Introduction

There is a multitude of individual, familial and social precursors of cocaine use in adulthood^1-4. Many studies report an independent association between early onset drinking and subsequent cannabis and cocaine use^1,2,5. However, one European study found that early drinking was not associated with current cocaine use⁶. That same study and many others have shown a relationship between early onset tobacco use and later drug abuse^5,7,8. Early onset cannabis use is also strongly associated with later cocaine use^2,9. There is ongoing debate regarding this relationship between early tobacco, alcohol and cannabis use and later use of other drugs. The ‘common liability model’ states that a combination of risk factors places some young people at increased risk of both early initiation and of subsequent progression to more serious and sustained drug abuse⁸. Fergusson, Boden & Horwood presented evidence that early onset cannabis use and alcohol use each acted as independent predictors of later drug abuse, while controlling for the psycho-social risk factors for early onset drinking and cannabis use². They, and others, argued that this is suggestive of a gateway role for each of these substances^10-12. Age of first alcohol use has decreased dramatically in Ireland over the past 50 years¹³. Alcohol intoxication has a direct effect in impairing risk perception, which may in turn increase the likelihood that a young person chooses to use cocaine. In a recent Irish survey, it emerged that 70% of the adolescents who had used cocaine were alcohol intoxicated on the occasion of first cocaine use¹⁴. While Europeans are the heaviest drinkers in the world, Irish adults binge drink more than any of their European counterparts¹⁵. Per capita alcohol consumption has increased by about 180% in Ireland over the past four decades, at a time when it was generally falling in Europe¹³. The ESPAD study indicates that smoking has reduced in adolescents in Ireland between 1995 and 2007¹⁶. Cannabis is the most widely used illicit drug with 20% of adolescents reporting lifetime use, and 9% reporting past month use in 2007¹⁶. Only 4% reported lifetime use of cocaine.

Although most international research points to a relationship between early onset drinking and later drug use, these studies have largely been conducted in the USA. There are few European studies examining this issue and one large multicentre study found no relationship between early onset drinking and later drug abuse in young adults⁶. Given the unusual drinking patterns demonstrated in Northern Europe, where heavy drinking is the norm and early drinking is widespread, it is possible that early onset drinking is unrelated to cocaine use in adulthood. Therefore, the aim of this study was to examine the relationship between cocaine use in young adulthood and early onset alcohol, cigarette and cannabis use.

Methods

National drug prevalence studies were conducted by the NACD in 2002 and 2006^17,18. The full questionnaire, along with a detailed description of multistage random sampling, is available in the respective technical reports^17,18. The response rates were 70% and 65%. We included all participants aged 20 to 29 years in this analysis. Pearson’s chi-squared test was used to test the association between a lifetime history of cocaine use and categorical variables. Median ages of first use were compared across groups using the Mann-Whitney test. A multivariable logistic regression was used to predict lifetime history of cocaine use using socio-demographic and other substance use variables. For tobacco and cannabis use, participants were divided into three categories: never users, those who used at the median age of first use for the sample or older and those who used younger than the median age. Age of first use of alcohol was available for 96% of the sample and was included as a covariate. The potentially non-linear relationship between age of first use of alcohol and lifetime history of cocaine was explored non-parametrically with restricted cubic splines¹⁹. Tests for non-linearity used the likelihood ratio test, comparing the model with only the linear term to the model with the linear and the cubic spline terms. An unadjusted restricted cubic spline with four knots and using the median age of first use of alcohol as a reference value was fitted to the data. Odds ratios (OR) and their 95% confidence intervals (CI) were calculated. Variables were selected using stepwise selection techniques and the significance level was set at a=0.05. Goodness of fit was assessed using the Hosmer-Lemeshow chi-squared test and Nagelkerke R squared.

Results

Of the 1,897 young adults interviewed, 57.1% were female. Lifetime use of alcohol in this sample was 95.6% and the median age of first use was 16 years (Interquartile range [IQR] 15 – 18). Lifetime use of tobacco was 61.9% and with a median age of first use of 16 years (IQR 14 – 17). Lifetime use of cannabis and cocaine was 31.2% and 7.0% respectively, with median ages of first use among users of 18 years (IQR 16–20) and 20 years (IQR 18 – 22) respectively. Among the 584 people with a lifetime history of cannabis use, there was only one person who had never drank alcohol, while there were 95 (16.3%) with no history of tobacco use. Use of cannabis preceded alcohol use in 27 (4.7%), cases and coincided with year of first drinking in a further 93 (16.1%) cases. The median time from drinking to cannabis use was 2 years (IQR 1 to 4 years). Among those with a history of both tobacco and cannabis use, there were 34 (7%) who used cannabis first and 77 (15.9%) cases where year of first use coincided. The median time from tobacco use to cannabis use was 2 years (IQR 1 to 4 years). All 131 people with a history of cocaine use drank alcohol. These individuals had a median age of first drinking of 15 years (IQR 14-16 years). Nobody used cocaine prior to commencement of drinking. The median gap from first drinking to cocaine use was 5 years (IQR 4-7 years). There were 8 (6%) cocaine users who commenced drinking at age 18 years or older. Among the 122 people who reported past use of both cocaine and cannabis, cocaine use never preceded cannabis use and there was a median time gap of 3 years (IQR 2 -5 years) from first cannabis use to first cocaine use. There were 19 (15%) cocaine users with no history of tobacco use. While tobacco use post dated cocaine use in 2 cases, the median time from first tobacco use to first cocaine use was 5 years (IQR 4 – 8 years). Table 1 compares people with lifetime use of cocaine to those with no use. Significant associations between a lifetime history of cocaine use and male gender, living in Dublin, lower levels of education, not living with a partner and early onset drinking, tobacco and cannabis use were observed. A higher rate of lifetime use of cocaine was observed in 2006 compared to 2002. In a multivariable logistic regression, survey year, relationship status, age of first alcohol use and history of cannabis use were significant predictors of lifetime use of cocaine (Table 2).

Due to the strong association between history of tobacco use and cannabis use (X2 (4) =254.5, p<0.001), with 65% of early cannabis users also early tobacco users, there was no independent effect for history of tobacco use in a multivariable model and the addition of history of tobacco use did not improve the model fit. The relationship between age of first alcohol use and history of cocaine use was non-linear (Figure 1) with the odds of history of cocaine use higher for those with first use of alcohol younger than the median age and a plateau effect for increasing age of first use of alcohol older than the median. The addition of non-linear terms improved the fit of the model. Increasing age of first use of alcohol was associated with a decrease in the odds of lifetime history of cocaine use (adjusted OR 0.82, 95% CI 0.73, 0.92).

Discussion

The route to cannabis and cocaine use in this Irish sample is quite typical of that seen in other international surveys⁹. Alcohol was a more prominent antecedent of both cannabis and cocaine use than tobacco. Unlike the other large European study which found that early onset tobacco, but not alcohol, was independently associated with later drug abuse⁶, we found the opposite. Early onset drinking and early cannabis use were independent predictors of lifetime cocaine use in these young Irish adults. Our finding is quite consistent with most of the international literature from outside of Europe^2,5,20. Our detected association with earlier onset drinking is particularly concerning in light of the declining age of drinking onset in Ireland over the past three decades and prevalent permissive attitudes to adolescent drinking by Irish parents^13,21. The association between age of alcohol initiation and cocaine use was non-linear, indicating that a one-year difference in timing of first drinking in early adolescence had a greater impact on odds of future cocaine use compared to a one-year difference in older adolescence. It is possible that our findings will not translate to other countries. Most drinkers in Ireland, demonstrate an unhealthy pattern of alcohol use²². Cocaine provides a different route to intoxication, and both substances are frequently used together²³. The majority of young people in Ireland who try cocaine are drunk at the time and this may not be the case in other countries¹⁴. Others have suggested that both alcohol and cannabis play a causal role in subsequent cocaine use^1,2,10,11. Our findings are consistent with this hypothesis. There is growing evidence that early onset drinking and cannabis use can cause enduring neuropsychological changes and some of these may explain the observed associations between early substance use and later escalation in drug use^11,24. A recent study has also demonstrated that early onset substance use tends to lead to assimilation into deviant and substance using peer groups which in turn leads to increased risk of escalation of substance use²⁵. Given the likely heterogeneity of routes to cocaine use for individuals in a population, it seems possible that early onset drinking and cannabis use may play a causal role in some cases, while simply being a proxy marker of risk in other cases⁴.

A strength of this study is the use of two nationally representative data sets. As a cross sectional study, no formal assessment of causality can be conducted. It is not possible to rule out the effect of unknown or unmeasured confounders. Recall bias and response bias might occur with respect to the onset of alcohol and drug use. The proportion of interviewees reporting lifetime use of cocaine was quite low and this limited the statistical modelling and contributed to fact that the 95% confidence intervals are quite wide. We conclude that adolescents who commence drinking in their early teenage years and those who use cannabis are at elevated risk of subsequent cocaine use. There are efforts in Ireland and elsewhere to reverse the downward trend in age of first drinking. If these efforts are successful, they may then also yield a reduction in cocaine use among young adults.

 

Conflict of interest:

No funding was received for the completion of this study. The authors have no financial conflicts of interest to declare.

 

Acknowledgements

We are grateful to the NACD for making the data available to the research team.

 

Correspondence: B Smyth, Department of Child Psychiatry, Children's Health Queensland Hospital, Brisbane, Australia

Email: [email protected]

 

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