Licorice and its potential beneficial effects in common oro-dental diseases

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1 (2012) 18, doi: /j x Ó 2011 John Wiley & Sons A/S All rights reserved REVIEW ARTICLE Licorice and its potential beneficial effects in common oro-dental diseases C Messier 1, F Epifano 2, S Genovese 2, D Grenier 1 1 Groupe de Recherche en E cologie Buccale, Faculte de Me decine Dentaire, Universite Laval, Quebec City, QC, Canada; 2 Dipartimento di Science del Farmaco, Universita` G. D Annunzio, Chieti Scalo, Chieti, Italy Licorice, the name given to the roots and stolons of Glycyrrhiza species, has been used since ancient times as a traditional herbal remedy. Licorice contains several classes of secondary metabolites with which numerous human health benefits have been associated. Recent research suggests that licorice and its bioactive ingredients such as glycyrrhizin, glabridin, licochalcone A, licoricidin, and licorisoflavan A possess potential beneficial effects in oral diseases. This paper reviews the effects of licorice and licorice constituents on both the oral microbial pathogens and the host immune response involved in common ora-dental diseases (dental caries, periodontitis, candidiasis, and recurrent aphthous ulcers). It also summarizes results of clinical trials that investigated the potential beneficial effects of licorice and its constituents for preventing treating oro-dental diseases. (2012) 18, Keywords: dental caries; periodontitis; candidiasis; recurrent aphthous ulcer; licorice in vitro and in vivo studies have suggested healthful properties of licorice and its bioactive constituents. More specifically, the pharmacological activities of licorice and their effects in the treatment for different human diseases such as cancer, atherosclerosis, gastric ulcers, hepatitis, bacterial infections, and immunodeficiency have been the topic of several reviews (Isbrucker and Burdock, 2006; Shen et al, 2007; Nassiri Asl and Hosseinzadeh, 2008). Evidence for a therapeutic application of licorice in oral diseases has also been reported. The aim of this manuscript is to review the effects of licorice extracts and their bioactive ingredients on both the oral microbial pathogens and the host immune response involved in common oro-dental diseases (dental caries, periodontitis, candidiasis, and recurrent aphthous ulcers). It also summarizes the results of clinical trials that investigated the potential beneficial effects of licorice constituents for preventing treating oral diseases. Pertinent papers were retrieved by searching MEDLINE via PubMed from 1970 to Introduction Licorice root (Radix Glycyrrhizae) is obtained from perennial plants native to Mediterranean countries, central to southern Russia, and certain regions of Asia. The genus name Glycyrrhiza is derived from the ancient Greek words glycos (meaning sweet) and rhiza (meaning root). Glycyrrhiza glabra L. and Glycyrrhiza uralensis Fisch. (Fam. Leguminosae) roots are the commonest sources of licorice used in cosmetics, foods, tobacco, and in both traditional and herbal medicine. Although inconsistent results were often obtained and or experimental design presented some deficiencies, numerous Correspondence: Daniel Grenier, Groupe de Recherche en E cologie Buccale, Faculté de Médecine Dentaire, Université Laval, 2420 rue de la Terrasse, Quebec City, G1V 0A6 QC, Canada. Tel: (418) , Fax: (418) , Daniel.Grenier@greb.ulaval.ca Received 3 March 2011; revised 15 June 2011; accepted 28 June 2011 Description of the plant, phytochemical composition, and safety While the genus Glycyrrhiza includes about 30 species (Nomura et al, 2002), the classic main botanical sources of Radix Glycyrrhizae, or licorice root, are G. glabra L. and G. uralensis Fisch. (Shen et al, 2007). Glycyrrhiza glabra is native to Mediterranean countries and certain regions of Asia. It is an herbaceous perennial shrub that grows up to 1 m in height. The licorice shrub has an extensive root system composed of a taproot and numerous stolons. The taproot, which has been harvested for medicinal uses as far back as 6000 years, is soft and fibrous and has a bright yellow interior. The stolons, which can reach 8 m in length, together with the taproot are the source of commercial licorice. Glycyrrhiza uralensis is a perennial herbaceous plant native to the Urals, Siberia, and the steppes and semidesertic regions of East Asia. The root system is similar to that of G. glabra.

2 Licorice is produced from the unpeeled, dried roots and stolons of G. glabra and G. uralensis. Phytochemically, both plants are among those well studied and contain several classes of secondary metabolites, the most abundant being saponins, flavonoids, isoflavonoids, chalcones, and coumarins as well as minor amounts of aurones, benzofurans, phenols, pterocarpans, and stilbenes. A detailed description of phytochemicals found in licorice is beyond the scope of this manuscript and have been previously reviewed (Wang et al, 2000; Kondo et al, 2007). Table 1 summarizes the major phytochemical classes identified in licorice extracts as well as their potential benefits in oro-dental diseases based on in vitro and in vivo studies. Most over-the-counter herbal remedies are relatively safe when used appropriately. However, some may have adverse effects or the potential to interfere with the Table 1 Major classes of phytochemicals found in licorice root (Radix Glycyrrhizae) and their potential beneficial effects in oral diseases Class Example Potential beneficial effects previously reported (references) Aurones Licoagroaurone None reported Benzofurans Licocoumarone None reported Chalcones Isoliquiritigenin Licochalcone A None reported Candidiasis (Messier and Grenier, 2011) Coumarins Glycyrol None reported Flavonoids Glabrol None reported Liquiritigenin Candidiasis (Lee et al, 2009) Isoflavonoids Glabridin, Candidiasis (Fatima et al, 2009; Messier and Grenier, 2011), periodontal disease Licoricidin (Choi, 2005) Periodontal disease (La et al, 2011) Licorisoflavan A Periodontal disease (La et al, 2011) Pterocarpenes Glycyrrhizol A Dental caries (He et al, 2006; Hu et al, 2011) Saponins Glycyrrhizin Candidiasis (Utsunomiya et al, 1999), dental caries (Segal et al, 1985; Gedalia et al, 1986; Sela et al, 1987; Deutchman et al, 1989; Steinberg et al, 1989; Goultschin et al, 1991; Söderling et al, 2006) Glycyrrhizic acid Dental caries (Edgar, 18b-Glycyrrhetinic 1978) Candidiasis (Pellati acid et al, 2009), periodontal disease (Sasaki et al, 2010) Stilbenes Gancaonin G Dental caries (He et al, 2006) therapeutic action of conventional drugs. Licorice and licorice constituents are known as generally recognized as safe for use in foods and over-the-counter drugs by the United States Food and Drug Administration (21 CFR ; ; ; ) (Isbrucker and Burdock, 2006). It is assumed that these products do not pose a health hazard, provided that they are not consumed in excess or by individuals who are sensitive to low levels of glycyrrhizin. Licorice root is available under various forms (candies, capsules, tablets, liquid extracts, etc.). Solid extract ( mg) consumed three times daily has been suggested for medicinal purposes (Touyz, 2009). Large amounts of licorice, more particularly glycyrrhizin, may cause severe hypertension, hypokalemia, and hypermineralocorticoid-like effects (Isbrucker and Burdock, 2006) through inhibition of 11b-hydroxysteroid dehydrogenase, which is responsible for the renal conversion of cortisol into cortisone (Quinkler and Stewart, 2003). This inhibition results in elevated levels of cortisol in the collecting duct of kidney, and potassium is excreted while sodium is retained, leading to hypertension. A linear dose-dependent rise in blood pressure has been reported for licorice consumption ( g daily for 2 4 weeks) corresponding to a daily intake of mg glycyrretinic acid (Sigurjonsdottir et al, 2001). These effects, which are reversible upon withdrawal of licorice or glycyrrhizin, may be more important for individuals using glucocorticoids on a regular basis. Several other adverse effects, including headache, premature birth, muscle weakness, and paralysis, have also been reported (Nassiri Asl and Hosseinzadeh, 2008). In addition, licorice has been shown to be a potent inhibitor of cytochrome CYP3A4 activity (Tsukamoto et al, 2005). Because this liver enzyme is responsible for metabolism of a wide range of drug molecules, a prolonged intake of high doses of licorice may affect the metabolism of coadministrated drugs, such as warfarin, hydrocortisone, and acetaminophen (Nassiri Asl and Hosseinzadeh, 2008). Dental caries and licorice Over the past 30 years, dental caries has declined in high socioeconomic populations but has increased in low income and fragile elderly populations. Dental caries is a complex and chronic disease that is associated with a progressive destruction of the hard tooth structures (enamel, dentin, cementum) by the action of acidogenic aciduric bacteria embedded in the dental plaque, also known as the dental biofilm. Although mutans streptococci (Streptococcus mutans and Streptococcus sobrinus) are the primary etiologic agents of dental caries, Lactobacillus spp., and Actinomyces spp. are also known to contribute to tooth decay (Takahashi and Nyvad, 2008, 2011). These bacteria metabolize exogenous dietary carbohydrates such as sucrose, glucose, and fructose and produce organic acids, mostly lactic acid, as by-products (Takahashi and Nyvad, 2008, 2011). When carbohydrates are present in excess, cariogenic bacteria can synthesize intra-cellular polysaccharides that can be utilized as an energy source to produce acids 33

3 34 when carbohydrates are limited. Through glucosyltransferases and fructosyltransferase activities, exopolysaccharides (glucans and fructans) are also produced by S. mutans and contribute to the formation of a dense and adherent biofilm that allows to concentrate acids on the tooth surface (Koo et al, 2009). Acid formation drives the dissolution of calcium and phosphate in the hydroxylapatite crystal structure of enamel, provided that the ph remains below the critical ph established at around 5.5. When fermentable carbohydrates are depleted, ph increases above the critical ph attributing to the buffer capacity of saliva. The remineralization process of the enamel then occurs through calcium and phosphate found in saliva. Therefore, the tooth is in a constant state of back-and-forth demineralization and remineralization processes. Decreasing the number of cariogenic bacteria, reducing acid production and preventing biofilm formation represent effective approaches for caries control. While the anti-cariogenic properties of licorice have been suggested for over 30 years, few studies on this aspect have been published. Glycyrrhizin, a sweettasting compound (50 times sweeter than sucrose) and the main triterpenoid saponin glycoside in G. glabra, has been the topic of several investigations. Segal et al (1985) showed that while the growth of S. mutans was not affected by glycyrrhizin in the presence of sucrose, its ability to adhere to a glass surface was almost completely inhibited. The anti-adherent property of glycyrrhizin was further studied by Sela et al (1987). These authors showed that glycyrrhizin dose-dependently inhibit the glucosyltransferase activity of S. mutans, which is involved in the formation of insoluble glucans required in biofilm formation. Interestingly, Gedalia et al (1986) reported that glycyrrhizin, when added to an acidulated phosphate-fluoride solution, increases fluoride uptake and reduces enamel solubility most likely due to a surface-coating effect and to its deposition in the porous structure of demineralized enamel. In contrast, Deutchman et al (1989) did not observe any significant effect with glycyrrhizin in regard to mineral loss in an artificial caries lesions in vivo model. This may have been related to insufficient glycyrrhizin concentration or exposure time. Glycyrrhizinic acid, the sugar-free form of glycyrrhizin, was also found to reduce enamel dissolution in vitro by inhibiting acid production by dental plaque (Edgar, 1978). Although the number of subjects involved was rather limited, some pilot clinical trials were performed on glycyrrhizin. A first study using a split-mouth design to apply glycyrrhizin (in the absence of oral hygiene procedures) was conducted on 21 subjects and showed that glycyrrhizin had a tendency toward a statistical significant effect for controlling dental plaque formation (plaque index score) after 3 4 days (Steinberg et al, 1989). Limitations of this study include lack of randomization, blinding, and a separate control group. In addition, the sample size may be too small to achieve statistical power. Another pilot randomized doubleblind study involving 40 subjects showed that toothbrushing (for up to 42 days) with a toothpaste containing glycyrrhizin (0.25% or 0.50%) had no effect on the plaque index in comparison with a control toothpaste (Goultschin et al, 1991). The authors suggested that the lack of effect may have been owing to an insufficient concentration of glycyrrhizin and or to a chemical incompatibility of glycyrrhizin with other ingredients contained in the toothpaste (Goultschin et al, 1991). In a more recent clinical study (16 subjects), So derling et al (2006) reported that toothbrushing three times a day with a gel containing % licorice extract (22% glycyrrhizin) for 2 weeks failed to reduce or change the composition of dental plaque compared to a control gel. However, in an in vivo acid production test, the licorice-containing gel was shown to inhibit acid production (So derling et al, 2006). Based on the above studies, it appears that glycyrrhizin, at concentrations and exposure times tested, offers few potential for caries control. Further randomized controlled trials are required prior to recommending for or against the use of glycyrrhizin in oral hygiene products. Four additional compounds isolated from G. uralensis have been reported to possess anti-bacterial activity against the cariogenic bacterium S. mutans (He et al, 2006). More specifically, the minimal inhibitory concentrations (MICs) for glycyrrhizol A, 6,8-diisoprenyl- 5,7,4 -trihydroxyflavone, glycyrrhizol B, and gancaonin G are 1, 2, 32, and 125 lg ml )1, respectively (He et al, 2006). Based on these observations, the same group prepared a licorice extract enriched in glycyrrhizol A and developed a sugar-free licorice-containing lollipop as a potential anti-caries product (Hu et al, 2011). Although the number of human subjects was limited, two pilot studies indicated that a 10-day use (twice daily) of licorice lollipops led to a marked reduction in salivary S. mutans (Hu et al, 2011). The authors recognized that these data are preliminary and that double-blind, randomized, longitudinal studies using more human subjects are required to conclude about the potential for using licorice lollipops for caries control in young children who are at risk for dental caries (Hu et al, 2011). Peters et al (2010) also carried out a pilot study using a lollipop made with a different licorice extract and showed that when used twice daily, it significantly reduces both number and relative percent of S. mutans in high-risk preschool children. However, this study suffered from the lack of a true control group in which a placebo was used. Further studies on licorice-containing lollipops need to demonstrate an anti-cavity effect clinically rather than just a modification of the oral microflora. While the above licorice lollipops may represent attractive anti-caries products for high-risk children and the elderly, users should be well informed about the potential adverse effects such as hypertension associated with excessive or prolonged use of such lollipops. Saliva is well known to possess several tooth-protective properties including anti-bacterial action, buffering capacity, cleansing effect, and remineralization activity (Dowd, 1999). Licorice being a sweet-tasting substance and acting as a gustatory stimulus may increase the

4 salivary flow and then provide a beneficial impact against dental caries. Periodontal diseases and licorice Periodontal diseases, which include gingivitis and periodontitis, are multifactorial chronic infections involving a specific group of Gram-negative anaerobic bacteria that interact with host immune cells. Gingivitis is characterized by an inflammation limited to the unattached gingiva, whereas periodontitis is a progressive, destructive disease that affects all supporting tissues of the teeth, including the alveolar bone. There are two major etiological factors involved in the pathogenesis of periodontitis. The first is the microbial component, notably the accumulation of periodontopathogenic bacteria in subgingival areas, which damage periodontal tissue through production of toxins and proteinases (O Brien-Simpson et al, 2004; Feng and Weinberg, 2006). The most putative pathogens associated with the chronic form of periodontitis are Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola while Aggregatibacter actinomycetemcomitans is strongly associated with the aggressive form. The second factor is the host response to the above periodontopathogens, notably the over-production by resident and immune cells of inflammatory mediators (pro-inflammatory cytokines and prostanoids) and matrix metalloproteinases (MMPs), which can modulate the progression and severity of periodontitis (Garlet, 2010). The treatment for periodontitis is aimed at removing dental plaque and calculus, as well as motivating patient s oral hygiene in order to control the dental biofilm. This can be achieved by mechanical, chemical, and surgical methods. Dental plaque and calculus can be removed by conventional scaling and root planing procedures. This non-surgical therapy combined with appropriate oral hygiene can help reduce tissue inflammation, pocket depth, and improve clinical periodontal attachment (Cobb, 2002; Suvan, 2005). The administration of local and systemic antibiotics, antiinflammatory drugs, or sub-anti-microbial low-dose doxycycline has been reported to provide additional benefits (Haffajee et al, 2003; Preshaw et al, 2004). Although no human clinical trials have been carried out, in vitro studies have brought evidence that licorice and its bioactive ingredients may represent potential phytochemicals for the development of a new natural therapy to treat or prevent periodontitis. Licorice can first act on periodontopathogens that represent the first etiological factor of the disease. Indeed, a crude extract from G. uralensis was reported to inhibit both the growth and biofilm formation by P. gingivalis (Bergeron et al, 2008), one of the key etiologic agent of chronic periodontitis. The effects of licorice on the host inflammatory response were also investigated. Macrophages and monocytes, found in high numbers in active periodontal sites compared to inactive sites (Zappa et al, 1991), play a crucial role in the inflammatory host response toward periodontal pathogens (Kornman et al, 1997). Indeed, the continuous high secretion of various cytokines and chemokines such as IL-1b, IL-6, IL-8, and TNF-a by these cells can modulate periodontal tissue destruction (Garlet, 2010). Bodet et al (2008) showed that human macrophages pretreated with a licorice extract prior to being stimulated with A. actinomycetemcomitans or P. gingivalis lipopolysaccharide (LPS) secrete significantly less pro-inflammatory cytokines (IL-1b, IL-6, IL-8, and TNF-a), indicating that the extract has an anti-inflammatory property. Licoricidin and licorisoflavan A, two major isoflavonoids isolated from the licorice extract, have been shown to be responsible for the anti-inflammatory effect (La et al, 2011). Interestingly, both molecules also inhibited MMP-7, -8, and -9 secretion by LPS-stimulated macrophages (La et al, 2011). The inhibition of pro-inflammatory cytokine and MMP secretion was associated with a reduced activation of transcription factor NF-jB p65 (Bodet et al, 2008; La et al, 2011), which plays a key role in the inflammatory response (Biesalski, 2007). Sasaki et al (2010) reported that 18b-glycyrrhetinic acid suppresses the LPS- and receptor activator of nuclear factor kappa-b ligand (RANKL)-induced phosphorylation of NF-jB p105 in vitro, which provides further support for the ability of licorice to modulate the inflammatory response. When administered either prophylactically or therapeutically in interleukin-10-deficient mice infected with a virulent strain of P. gingivalis, 18b-glycyrrhetinic acid markedly reduced alveolar bone loss. 18b-Glycyrrhetinic acid appears to inhibit the severity of periodontitis in a mouse model by inactivating the transcription factor NF-jB in an IL-10- and glucocorticoid-independent fashion (Sasaki et al, 2010). The resorption of alveolar bone is a typical hallmark of periodontal disease. Alveolar bone destruction is mediated by the recruitment and differentiation of osteoclasts into their mature phenotype (Teitelbaum, 2000). Once activated, resorptive osteoclasts attach to the bone surface and promote mineral dissolution by acidification of the sub-osteoclastic microenvironment. Subsequently, the demineralized organic matrix of bone is degraded by osteoclast-derived MMPs. Accordingly, the modulation of osteoclast formation and function is pointed as one of the therapeutic targets in the prevention of alveolar bone loss associated with periodontal disease. Recently, our laboratory showed that the isoflavonoid glabridin can inhibit the RANKL-dependent differentiation of human osteoclast precursors into mature osteoclast (V.D. La, J. Santos, S.I. Tanabe, Grenier unpublished data). In addition, glabridin was also found to reduce bone degradation mediated by mature osteoclasts in an in vitro model (unpublished data). Although in vivo studies are required, the above data suggest that glabridin can interfere with osteoclastic cell maturation and functions and thus has a valuable potential for therapeutic application in the treatment and prevention of bone loss related to inflammatory disorders as the periodontal disease. Interestingly, Choi (2005) reported that glabridin has a direct stimulatory effect on bone formation by enhancing the proliferation of osteoblasts (bone-forming cells) as well as their ability to synthesize collagen. 35

5 36 Oral candidiasis and licorice Oral candidiasis is an opportunistic infection of the oral cavity caused by an overgrowth of Candida species, the most common being Candida albicans (McCullough and Savage, 2005; Samaranayake et al, 2009). This yeast is a normal commensal microorganism in the mouth and generally causes no infections in healthy people. Several factors that induce changes in the oral environment can predispose to oral candidiasis and include antibiotics, corticosteroids, xerostomia, diabetes mellitus, nutritional deficiencies, and immunosuppressive diseases and therapy (Zunt, 2000). The diagnosis of oral candidiasis is usually based on recognition of its specific clinical pattern (Zunt, 2000). The most common forms of oral candidiasis are pseudomembranous candidiasis (also called thrush) and erythematous candidiasis, which includes denture stomatitis. Pseudomembranous candidiasis is characterized by creamy-white pseudomembranes consisting of superficial mucosal cells, neutrophils, and fungal hyphae. It is localized on the surface of the labial and buccal mucosa, hard and soft palate, tongue, periodontal tissue, and oropharynx. Denture stomatitis, which can develop in complete or partial denture wearers, is an inflammation of the palatal mucosa that is in contact with the denture (localized or generalized) (McCullough and Savage, 2005; Samaranayake et al, 2009). The treatment for oral candidiasis is based on appropriate antifungal drugs (topical or systemic), appropriate oral hygiene, and regular disinfection of any removable intra-oral prosthetic appliances (Zunt, 2000). While the physiological status of the host is a critical factor governing the initiation of oral candidiasis, the pathogenic potential of C. albicans is also of utmost importance. This pathogen has developed an effective range of virulence factors and strategies to colonize the host, overcome host immune defenses, and cause tissue damage (Yang, 2003). More specifically, C. albicans virulence has been related to its filamentous form following a phenotypic transition, also known as switch, from blastospores to hyphae (Kumamoto and Vinces, 2005). The ability of C. albicans to form biofilms may also be a key element in its virulence because it has been reported that in a biofilm, C. albicans may be up to 1000 times more resistant to antifungal drugs compared to its planktonic form (Hawser and Douglas, 1995). Interestingly, it was previously reported that the morphological switch from blastospores to hyphal cells is important in many processes, such as biofilm formation (Krueger et al, 2004). Few studies have investigated the effect of licorice on C. albicans. Motsei et al (2003) reported the antifungal effect of organic solvent extracts of G. glabra on C. albicans. A more recent in vitro study showed that glabridin has potent activity against amphotericin B resistant strains of C. albicans, with MIC values of lg ml )1 (Fatima et al, 2009). 18b-Glycyrrhetinic acid, another compound isolated from G. glabra, reduced the growth of C. albicans in a ph-dependent manner at relatively low concentrations (6.25 lg ml )1 ) (Pellati et al, 2009). In a recent study, Messier and Grenier (2011) investigated the effects of two licorice polyphenolic compounds (licochalcone A and glabridin) on the growth, killing, biofilm formation, and adherence of C. albicans. The MICs of both compounds were lg ml )1, while the minimal fungicidal concentration was 12.5 lg ml )1 for glabridin and 100 lg ml )1 for licochalcone A. Both compounds also acted in synergy with nystatin to inhibit the growth of C. albicans. The effect of licorice constituents on virulence properties of C. albicans was also investigated. Biofilm formation was inhibited by 30 80% with licochalcone A at lg ml )1, while glabridin had no effect. A strong inhibitory effect (60 100%) on the transition blastospore-hyphal form was observed with 100 lg ml )1 licochalcone A and glabridin. All together, these findings suggest that licochalcone A and glabridin show promise as a therapeutic agents for treating oral C. albicans infections. Such a capacity of these molecules to act as inhibitors of virulence factors represents alternative and innovative pathways of chemotherapy for pathogens that are resistant to classical anti-microbial agents. C. albicans may not easily develop resistance to such molecules that target the virulence process. Glycyrrhizin was found to improve the resistance of thermally injured mice against C. albicans infections by inducing CD4+ Th1 cells that suppress cytokine production by burn-associated cells (Utsunomiya et al, 1999). Furthermore, animal studies have shown that liquiritigenin, a licorice flavonoid, has immunomodulating activity and can protect mice against disseminated candidiasis by acting on the CD4+ Th1 immune response (Lee et al, 2009). Recurrent aphthous ulcers and licorice Recurrent aphthous ulcers are among the most common oral mucosal disease encountered in children and adults. Three types of recurrent aphthous ulcers have been described: minor (the most common), major, and herpetiform (Zunt, 2001; Mun oz-corcuera et al, 2009). While minor aphthous ulcers are small and cause the least discomfort, major aphthae are more severe, larger, slower to heal and more painful. Herpetiform ulcers occur as multiple lesions (up to 100) and can range from 1 to 3 mm in diameter each. The etiology of these oral ulcerations is very diversified and includes systemic, local, and immune factors, bacterial and viral agents as well as food and chemical sensitivities (Scully et al, 2003; Mun oz-corcuera et al, 2009). The diagnosis of recurrent aphthous ulcers is primarily clinical, although additional diagnostic laboratory tests (cytology, biopsy, culture) may be required if ulcers fail to heal. Palliative care is provided for recurrent aphthous ulcers in order to relieve pain, promote healing, and prevent secondary infection (Zunt, 2001). Some reports on the effect of licorice for controlling the pain and reduce the healing time of aphthous ulceration have been published. In a very preliminary study involving 20 subjects, Das et al (1989) reported that the use of a mouthwash containing a deglycerinized

6 licorice extract for 2 weeks tends to provide pain relief and accelerate the healing of aphthous ulcers. In a more recent study, Moghadamnia et al (2009) investigated the efficacy of licorice bioadhesive hydrogel patches to promote healing and pain relief. Their results showed that biopatches containing licorice are almost equally effective as the control patches without licorice, suggesting that the mechanical mucosal protection alone was important in reducing pain and promoting healing. This study suffered from the low number of subjects included (15) and the fact that only one concentration of licorice was tested (1%). On the contrary, in a randomized, double-blind clinical trial (23 subjects) using a dissolving oral patch containing a licorice extract for up to 8 days, Martin et al (2008) observed an improvement in ulcer size and pain compared to the use of a placebo patch. Taken together, the above studies suggest that additional research is required to conclude about the potential benefits of licorice in recurrent aphthous ulcers. Conclusions Recent research suggests that licorice extracts and licorice bioactive ingredients such as glabridin, licoricidin, licorisoflavan A, licochalcone A, and glycyrrhizin have potential beneficial effects in oral diseases. These effects have been associated with the anti-adherence, anti-microbial, and anti-inflammatory properties of the compounds. Because some oral diseases have a complex etiology, which involves microbial pathogens and the host immune response, agents with dual functionality such as licorice phytochemicals may offer a therapeutic advantage. The strength of using phytochemicals as anti-microbials is that they are structurally different from classical microbially derived antibiotics. Therefore, it is likely that they have modes of action distinct from existing compounds and for which microbial pathogens did not develop resistance. On the one hand, in vitro studies suggested the potential of licorice and its bioactive constituents for the management of oral diseases, more particularly periodontal diseases. On the other hand, inconsistent results were often obtained in the human clinical trials. Several of the in vivo studies presented deficiencies in regard to experimental design and the number of subjects included making it difficult to achieve statistical power. Therefore, licorice extracts and licorice constituents incorporated into oral hygiene products such as mouthwash, toothpaste, gel, and chewing gum need to be further investigated in clinical studies in order to validate the beneficial effects observed in in vitro assays. Considering the possible adverse effects, especially hypertension, of a prolonged intake of high doses of licorice, a localized application of these bioactive substances may be more appropriate. For instance, insertion into diseased periodontal sites of a licoricecontaining resorbable fiber permitting the slow release of the active ingredients could locally act on periodontopathogens and the host inflammatory response, the two factors involved in periodontal tissue destruction. Acknowledgements Funding for licorice research was provided by the Ministe` re du De veloppement Économique de l Innovation et de l Exportation to D.G. Author contributions All authors contributed equally to the manuscript. References Bergeron C, Bodet C, Gafner S, Michaud A, Dumas L, Grenier D (2008). Effects of licorice on Porphyromonas gingivalis growth and biofilm viability. J Dent Res 87 (Special Issue B): abstract www. dentalreasearch.org [accessed on 10 August 2011]. Biesalski HK (2007). Polyphenols and inflammation: basic interactions. Curr Opin Clin Nutr Metab Care 10: Bodet C, La VD, Gafner S, Bergeron C, Grenier D (2008). A licorice extract reduces lipopolysaccharide-induced proinflammatory cytokine secretion by macrophages and whole blood. J Periodontol 79: Choi EM (2005). The licorice root derived isoflavan glabridin increases the function of osteoblastic MC3T3-E1 cells. Biochem Pharmacol 70: Cobb CM (2002). Clinical significance of non-surgical periodontal therapy: an evidence-based perspective of scaling and root planing. J Clin Periodontol 29: Das SK, Das V, Guati AK, Singh VP (1989). Deglycyrrhizinated liquorice in aphthous ulcers. J Assoc Physicians India 37: 647. Deutchman M, Petrou ID, Mellberg JR (1989). Effect of fluoride and glycyrrhizin mouthrinses on artificial caries lesions in vivo. Caries Res 23: Dowd FJ (1999). Saliva and dental caries. Dent Clin North Am 43: Edgar WM (1978). Reduction in enamel dissolution by liquorice and glycyrrhizinic acid. J Dent Res 57: Fatima A, Gupta VK, Luqman S et al (2009). Antifungal activity of Glycyrrhizia glabra extracts and its active constituent glabridin. Phytother Res 23: Feng Z, Weinberg A (2006). Role of bacteria in health and disease of periodontal tissue. Periodontol : Garlet GP (2010). Destructive and protective roles of cytokines in periodontitis: a re-appraisal from host defense and tissue destruction viewpoints. J Dent Res 89: Gedalia I, Stabholtz A, Lavie A, Shapira L, Pisanti S, Segal R (1986). The effect of glycyrrhizin on in vitro fluoride uptake by tooth enamel and subsequent demineralization. Clin Prev Dent 8: 5 9. Goultschin J, Palmon S, Shapira L, Brayer L, Gedalia I (1991). Effect of glycyrrhizin-containing toothpaste on dental plaque reduction and gingival health in humans. J Clin Periodontol 18: Haffajee AD, Socransky SS, Gunsolley JC (2003). Systemic anti-infective periodontal therapy. A systematic review. Ann Periodontol 8: Hawser SP, Douglas LJ (1995). Resistance of Candida albicans biofilm to antifungal agents in vitro. Antimicrob Agents Chemother 39: He J, Chen L, Heber D, Shi W, Lu QY (2006). Antibacterial compounds from Glycyrrhiza uralensis. J Nat Prod 69:

7 38 Hu CH, He J, Eckert R et al (2011). Development and evaluation of a safe and effective sugar-free herbal lollipop that kills cavity-causing bacteria. Int J Oral Sci 3: Isbrucker RA, Burdock GA (2006). Risk and safety assessment on the consumption of licorice root (Glycyrrhiza sp.), its extract and powder as a food ingredient, with emphasis on the pharmacology and toxicology of glycyrrhizin. Regul Toxicol Pharmacol 26: Kondo K, Shiba M, Nakamura R, Morota T, Shoyama Y (2007). Constituent properties of licorices derived from Glycyrrhiza uralensis, G. glabra, or G. inflata identified by genetic information. Biol Pharm Bull 30: Koo H, Xiao J, Klein MI (2009). Extracellular polysaccharides matrix an often forgotten virulence factor in oral biofilm research. Int J Oral Sci 1: Kornman KS, Page RC, Tonetti MS (1997). The host response to the microbial challenge in periodontitis: assembling the players. Periodontol : Krueger KE, Ghosh AK, Krom BP, Cihlar RL (2004). Deletion of the NOT4 gene impairs hyphal development and pathogenicity in Candida albicans. Microbiology 150: Kumamoto CA, Vinces MD (2005). Contributions of hyphae and hypha-co-regulated genes to Candida albicans virulence. Cell Microbiol 7: La VD, Tanabe S, Bergeron C, Gafner S, Grenier D (2011). Modulation of matrix metalloproteinase and cytokine production by licorice isolates licoricidin and licorisoflavan A: potential therapeutic approach for periodontitis. J Periodontol 82: Lee JY, Lee JH, Park JH et al (2009). Liquiritigenin, a licorice flavonoid, helps mice resist disseminated candidiasis due to Candida albicans by Th1 immune response, whereas liquiritin, its glycoside form, does not. Int Immunopharmacol 5: Martin MD, Sherman J, van der Ven P, Burgess J (2008). A controlled trial of a dissolving oral patch concerning glycyrrhiza (licorice) herbal extract for the treatment of aphthous ulcers. Gen Dent 56: McCullough MJ, Savage NW (2005). Oral candidosis and the therapeutic use of antifungal agents in dentistry. Aust Dent J 50: S36 S39. Messier C, Grenier D (2011). Effect of licorice compounds licochalcone A, glabridin and glycyrrhizic acid on growth and virulence properties of Candida albicans. Mycoses In press. Moghadamnia AA, Motallebnejad M, Khanian M (2009). The efficacity of the bioadhesive patches containing licorice extract in the management of recurrent aphthous stomatitis. Phytother Res 23: Motsei ML, Lindsey KL, van Staden J, Ja ger AK (2003). Screening of traditionally used South African plants for antifungal activity against Candida albicans. J Ethnopharmacol 86: Mun oz-corcuera M, Esparza-Go mez G, Gonza lez-moles MA, Bascones-Martínez A (2009). Oral ulcers: clinical aspects. A tool for dermatologists. Part I. Acute ulcers. Clin Exp Dermatol 34: Nassiri Asl M, Hosseinzadeh H (2008). Review of pharmacological effects of Glycyrrhiza sp. and its bioactive compounds. Phytother Res 22: Nomura T, Fukai T, Akiyama T (2002). Chemistry of phenolic compounds of licorice (Glycyrrhiza species) and their estrogenic and cytotoxic activities. Pure Appl Chem 74: O Brien-Simpson NM, Veith PD, Dashper SG, Reynolds EC (2004). Antigens of bacteria associated with periodontitis. Periodontol : Pellati D, Fiore C, Armanini D, Rassu M, Bertoloni G (2009). In vitro effects of glycyrrhetinic acid on the growth of clinical isolates of Candida albicans. Phytother Res 23: Peters MC, Tallman JA, Braun TM, Jacobson JJ (2010). Clinical reduction of S. mutans in pre-school children using a novel liquorice root extract lollipop: a pilot study. Eur Arch Paediatr Dent 11: Preshaw PM, Hefti AF, Novak MJ et al (2004). Subantimicrobial dose doxycycline enhances the efficacy of scaling and root planing in chronic periodontitis: a multicenter trial. J Periodontol 75: Quinkler M, Stewart PM (2003). Hypertension and the cortisol-cortisone shuttle. J Clin Endocrinol Metab 88: Samaranayake LP, Keung Leung WK, Jin L (2009). Oral mucosal fungal infections. Periodontol : Sasaki H, Suzuki N, AlShwaimi E et al (2010). 18b-Glycyrrhetinic acid inhibits periodontitis via glucocorticoid-independent nuclear factor-jb inactivation in interleukin-10-deficient mice. J Periodontal Res 45: Scully C, Gorsky M, Lozada-Nur F (2003). The diagnosis and management of recurrent aphthous stomatitis: a consensus approach. J Am Dent Assoc 134: Segal R, Pisanty S, Wormser R, Azaz E, Selan MN (1985). Anticariogenic activity of licorice and glycyrrhizine I: inhibition of in vitro plaque formation by Streptococcus mutans. J Pharm Sci 74: Sela MN, Steinberg D, Segal R (1987). Inhibition of the activity of glucosyltransferase from Streptococcus mutans by glycyrrhizin. Oral Microbiol Immunol 2: Shen XP, Xiao PG, Liu CX (2007). Research and application of Radix Glycyrrhizae. Asian J Drug Metab Pharmacokinet 7: Sigurjonsdottir HA, Franzson L, Manhem K, Ragnarsson J, Sigurdsson G, Wallerstedt S (2001). Liquorice-induced rise in blood pressure: a linear dose-response relationship. J Hum Hypertens 15: So derling E, Karjalainen S, Lille M, Maukonen J, Saarela M, Autio K (2006). The effect of liquorice extract-containing starch gel on the amount and microbial composition of plaque. Clin Oral Invest 10: Steinberg D, Sgan-Cohen HD, Stabholz A, Pizanty S, Segal R, Sela MN (1989). The anticariogenic activity of glycyrrhizin: preliminary clinical trials. Isr J Dent Sci 2: Suvan JE (2005). Effectiveness of mechanical nonsurgical pocket therapy. Periodontol : Takahashi N, Nyvad B (2008). Caries ecology revisited: microbial dynamics and the caries process. Caries Res 42: Takahashi N, Nyvad B (2011). The role of bacteria in the caries process: ecological perspectives. J Dent Res 90: Teitelbaum SL (2000). Bone resorption by osteoclasts. Science 289: Touyz LZG (2009). Liquorice health check, oro-dental implications, and a case report. Case Report Med 2009: , doi: /2009/ Tsukamoto S, Aburatani M, Yoshida T, Yamashita Y, El- Beih AA, Ohta T (2005). CYP3A4 inhibitors isolated from licorice. Biol Pharm Bull 28:

8 Utsunomiya T, Kobayashi M, Herndon DN, Pollard RB, Suzuki F (1999). Effects of glycyrrhizin, an active component of licorice roots, on Candida albicans infection in thermally injured mice. Clin Exp Immunol 116: Wang ZY, Athar M, Bickers DR (2000). Licorice in foods and herbal drugs: chemistry, pharmacology, toxicology and uses. In: Mazza G, Oomah BD, eds. Herbs, botanicals & teas. Technomic Publishing Co. Inc, Lancaster, PA, pp Yang YL (2003). Virulence factors of Candida species. J Microbiol Immunol Infect 36: Zappa U, Reinking-Zappa M, Graf H, Espeland M (1991). Cell populations and episodic periodontal attachment loss in humans. J Clin Periodontol 18: Zunt SL (2000). Oral candidiasis: diagnosis and treatment. J Pract Hyg 9: Zunt SL (2001). Recurrent aphthous ulcers: prevention and treatment. J Pract Hyg 10: