Cannabis use and adverse outcomes in young people: Summary Report CAYT Impact Study: Report No. 7 Sally Bridges, Julia Hall and Chris Lord with Hashim Ahmed and Linda Maynard 1
The Centre for Analysis of Youth Transitions (CAYT) is an independent research centre with funding from the Department for Education. It is a partnership between leading researchers from the Institute of Education, the Institute for Fiscal Studies, and the National Centre for Social Research. 2
Contents 1. Table of tables 3 2. Introduction 4 3. Methodologies 4 4. Prevalence 5 5. Factors associated with cannabis use 5 6. Early users 6 7. Extent or intensity of use 7 8. Trajectories of early cannabis users 8 9. Education and employment 9 10. Criminality 9 11. Cannabis as a gateway drug 9 12. Adverse outcomes of cannabis use 9 1. Table of tables Table 1 Frequency of cannabis use by whether still using cannabis at wave 4 (base: early cannabis users)... 8 3
2. Introduction Cannabis is the most widely used illegal drug in Britain, particularly amongst young people. This report is part of a wider research project commissioned by DfE through the Centre for Analysis of Youth Transitions CAYT and aims to increase knowledge and understanding of teenage cannabis use and associated adverse outcomes. This summary report presents key findings from three studies of cannabis use among young people: a review of the international literature and two secondary analyses of large survey datasets; the Offending, Crime and Justice Survey (OCJS) and the Avon Longitudinal Survey of Parents and Children (ALSPAC). The key research questions for this programme of work were: How prevalent is cannabis use among young people? What are the health, educational and social harms associated with cannabis use among under 18 year olds and to what extent can causality be established? What are the main risk factors associated with cannabis use among young people, particularly early and heavy cannabis use, what is the nature of these associations, and to what extent can causality be established? What is the relationship between young people s cannabis use and subsequent drug use and dependency? Does the age of onset of cannabis use increase the harms or risk of harm experienced? What are the typical trajectories of early cannabis users as they move through and out of adolescence? The full reports for the three studies are available as annexes to this summary report 1. 3. Methodologies Complete methodological details are provided in the respective full reports, with key details summarised here. 1 Cannabis Use and Adverse Outcomes in Young People: A literature review. Sally Bridges and Linda Maynard. 2013. CAYT Report 7a. Cannabis Use and Adverse Outcomes in Young People: Analysis of the Offending, Crime and Justice Survey. Chris Lord. 2013. CAYT Report 7b What predicts young people using cannabis? Analysis of the Avon longitudinal study of parent and young people. Julia Hall with Hashim Ahmed. 2013. CAYT Report 7c 4
The literature review was based on international scholarship published in peer reviewed journals over the last 10 years. As the main aim was to identify causal links between cannabis use and adverse outcomes, the focus was on longitudinal and cohort studies. The method used was a Rapid Evidence Assessment, which is a compressed and delineated version of a Systematic Review. Evidence was harvested from eight online databases. The Offending, Crime and Justice Survey (OCJS) is a national, longitudinal, self-report survey carried out in England and Wales. Data from four waves of the survey conducted between 2003 and 2006 were analysed using logistic regression methods and controlling for potential confounding factors. The sample included respondents aged 10-25 at wave 1 who took part in all four waves of the survey. The Avon Longitudinal Survey of Parents and Children (ALSPAC) follows children born in 1991 and 1992 to mothers living in the former county of Avon. The most recent data available were collected from teenagers at age 16. As with the OCJS, analyses were conducted using logistic regression techniques. 4. Prevalence While cannabis is known to be the most commonly used illegal drug in the UK, evidence from the ALSPAC show that the majority (73%) of young people have not tried it by the time they are sixteen and a half years. The figure is lower in the OJCS sample where 52% of the 10-25 year olds had never used cannabis. Data showed that when young people do try cannabis, the majority stop using it later in adolescence. 17% of young people in the ALSPAC sample had first tried cannabis aged 15 years or under, however, over three quarters (76%) of these had ceased their use by the age of 16.5 years. In terms of frequency of use, in ALSPAC, of those who had tried cannabis (27%), 45% had tried it less than 5 times, 26% 5-19 times and 28% 20 or more times. In interpreting these findings, it is important to bear in mind that they reflect the selfreports of young people. 5. Factors associated with cannabis use The two secondary analyses of longitudinal studies identified a range of factors that were associated with a young person having used cannabis. Logistic regression techniques were used to isolate specific factors and control for potential confounding variables. This provides an indication of the strength of the relationship or association between cannabis use and each factor. Logistic regression cannot on its own confirm the presence of a causal relationship or the direction of association. It is often feasible that a factor which might play a role in causing someone to use cannabis could also be caused by cannabis use (truanting, for example). This can be mitigated to a certain extent by the use of 5
longitudinal data which allow for variables that are separated across time however, it is still not a measure of causality. From the ALSPAC study, the factors most strongly associated with ever having tried cannabis were found to be: Cigarette smoking, especially before age 15 and especially if relatively heavy. This is the strongest predictor (those who had first smoked a cigarette before the age of 15 were 28 times more likely to try cannabis than those who had not). Use of other hard drugs Having a parent who has been in trouble with the police Mother having used cannabis prior to, during or after pregnancy Mother having used alcohol (and heavier use = a stronger association) Mothers educational qualifications (although the pattern is somewhat irregular Children of mothers with O level qualifications were least likely to have ever tried cannabis (23%) and the highest proportion to have tried cannabis was amongst children of mothers with university degrees (32%). Children whose mothers had no educational qualifications were most likely to first try cannabis under the age of 15 (22%) Where available, the mother s partner also filled in a questionnaire and the following factors were found to be significant predictors of the child s cannabis use: Mother s partner 2 having used cannabis since child born Mother s partner having been self-employed or economically inactive (compared to those employed full or part-time) during mother s pregnancy. 6. Early users Analysis of the OCJS focused on those young people who started using cannabis early, at age 15 or younger, comparing them to both non-users and to respondents who had tried cannabis for the first time at age 16 or older. As outlined in the literature review, there is research to indicate that early users of cannabis are more likely to have worse educational outcomes including school completion and early school leaving, university enrolment and university degree attainment. Early cannabis users were: More likely to be of White British ethnic origin Less likely to have been brought up by both their natural parents 2 Mother s partner may or may not be the child s natural parent. 6
More likely to have grown up in rented accommodation This study found that the factors predicted by early cannabis use were: 3 Truanting from school Criminal or antisocial behaviour Use of Class A drugs Self-perceived poor health Being unhappy. Again, it is important to remember that any potentially causal relationships could run in either direction. Outcomes for early cannabis users who continued to use the drug post-16 were compared to those who stopped using post-16 and found that: Just over half of early users had stopped taking cannabis by wave 4 of the survey The majority of early users who were in employment by wave 4 had stopped taking cannabis Frequency of early cannabis use is related to later use, with the majority of nonfrequent users having stopped taking cannabis. Early cannabis users who had stopped were less likely to use Class A drugs Early users who had stopped were less likely to have committed an offence, a non-minor offence or a drug offence in wave 4. 7. Extent or intensity of use Like many other studies of cannabis use, both the OCJS and the ALSPAC measured how frequently young people had used cannabis. This follows research literature which has shown a dose-response relationship between cannabis use and poor educational attainment (see literature review for details of these studies). However, other aspects of extent or intensity of use, for example, dose per use or strength of active agent were not measured in these studies or in any of those reviewed. Figure 1 shows the frequency of cannabis use among the 28 per cent of ALSPAC respondents who had tried cannabis at least once by age 16. It shows that while almost half (45 per cent) of those who had tried it had done so less than five times, more than a quarter (28 per cent) had used it 20 times or more. 3 The OCJS did not collect information on cigarette smoking, which many studies including ALSPAC have found to be very strongly associated with cannabis use. 7
Figure 1 Number of times tried cannabis by age 16 (base: those who had tried it at least once; n= 2191 ) Tried fewer than 5 times Tried 5 to 19 times Tried at least 20 times Source: ALSPAC 8. Trajectories of early cannabis users Just over half of those OCJS respondents who reported having first used cannabis at age 15 or younger had not used it in the 12 months preceding the wave 4 survey, 4 years later (56 per cent). As shown in Figure 3, findings from the OCJS also indicate that continued cannabis use among early cannabis users is influenced by the frequency of their cannabis use: early users who were still using at wave 4 were more likely to have used frequently (60 per cent) than those who had stopped using by wave 4 (38 per cent). Table 1 Frequency of cannabis use by whether still using cannabis at wave 4 (base: early cannabis users) Frequency of cannabis use Cannabis use at wave 4 Frequent Once a month Not frequent Total Still using 60% [6%] 33% 100% Stopped using 38% [8%] 54% 100% Sample size 88 13 78 179. Source: OCJS 2003-2006 8
9. Education and employment Those early cannabis users who were in employment by wave 4 were more likely to have stopped using cannabis (62 per cent) than those who were in education (43 per cent), although 57 per cent of those in the unemployed or inactive category had also stopped. 10. Criminality Findings from the OCJS indicate that early cannabis users who were still using cannabis at wave 4 of the survey were: Twice as likely as those who had stopped to have committed a criminal offence Twice as likely as those who had stopped to have committed a non-minor offence Seven times as likely as those who had stopped to have committed a drug offence. 11. Cannabis as a gateway drug As already reported, both the OCJS and the ALSPAC found a strong association between use of cannabis and use of hard drugs, and the OCJS also indicated that early users of cannabis were especially likely to go on to use Class A drugs subsequently, even when some potential confounding factors were controlled for. 4 Forty per cent of OCJS respondents who had first taken cannabis at age 15 or younger had subsequently gone on to use a Class A drug by wave 4 of the survey, and early users who were still using cannabis at wave 4 were more likely than those who had stopped to have used a Class A drug. The literature review concluded that further research is required to assess whether there is a causal relationship between cannabis use and use of harder drugs. 12. Adverse outcomes of cannabis use The international literature review considered the empirical evidence relating to what are commonly thought to be some of the key adverse outcomes of cannabis use among young people, and reflected on the implications of this for a range of hypotheses regarding the nature of the relationships between cannabis use and these outcomes. It focused on longitudinal and cohort studies because, by showing the order in which phenomena arise over time, these provide the best indication of likely cause and effect (though there are still important limitations). The main findings of the literature review can be summarised as follows: 4 The factors controlled for were gender, ethnicity and whether the young person was brought up by both natural parents, just one natural parent, a natural parent and a step-parent, or neither natural parent. 9
A large body of evidence suggests that cannabis use in young people is linked to an increased risk of a range of negative health, educational and social outcomes, especially psychosis, affective disorders such as anxiety and depression, and poor educational performance. These risks generally appear to be elevated in those who start using cannabis at a younger age, and those who use it more frequently. There is some evidence to suggest that cannabis use itself leads to some negative outcomes. However, outcomes are different for different users not all young people who use cannabis go on to experience negative outcomes. There is evidence that, in some cases, a separate risk factor may underlie both cannabis use and the negative outcomes with which it is associated. For example it may be that certain environmental factors such as unemployment or deprivation, could lead someone both to use cannabis and to develop mental health problems. While different studies control for different factors in testing the strength of associations between cannabis use and negative outcomes, the possible presence of additional confounds is always hard to rule out. In addition, there is evidence that particularly with regard to mental health, some people might possess a particular predisposition towards experiencing negative outcomes as a result of cannabis use. For example, it may be that some people are genetically or biologically predisposed to react to cannabis in a certain way that makes them more likely to develop schizophrenia as a result of using cannabis. This might explain why some people experience negative outcomes from cannabis use while others do not. There is also some support for the argument that there is a period during adolescence (whilst the adolescent brain is in a state of development) when individuals may be particularly affected by cannabis use (those whose use more cannabis being at more risk of negative outcomes) In terms of educational outcomes, there is some evidence that cannabis use may play a role in poor educational performance but this may also be due to some unmeasured factors. It could also be that cannabis use occurs within peer groups who encourage young people to take on other non- conventional behaviours (early sexual activity, unplanned parenthood, unemployment, etc.) and to leave school early. It could also be that cannabis use has a more immediate effect. For example, if a young person is intoxicated and unable to perform well in an exam situation. Two other common hypotheses regarding the nature of the relationships between cannabis use and negative outcomes receive less support from the literature. 10
These are the self-medication hypothesis (that people use cannabis as a response to adverse conditions or situations) and the other drugs hypothesis (use of other drugs strongly associated with use of cannabis actually cause the negative outcome). Overall, the nature of the relationships between cannabis use and various adverse outcomes is far from clear and seems likely to vary according to the specific outcome or type of outcome. Further well-controlled longitudinal research is needed to explore these relationships. Further neuro-physiological research would also be helpful in examining the hypothesis that some young people have a genetic or biological predisposition towards certain negative outcomes from cannabis use. This can also be used to examine the biological and/or genetic reasons that may explain why using cannabis at a younger age appears to carry a greater risk of negative outcomes. 11