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Cannabis and other drug use


Cannabis use is common, particularly among the young. Cannabis has also been seen as leading users into use of 'harder' drugs, like cocaine or heroin. There are some studies reporting early use of cannabis as a risk factor for other drugs and drug-related problems. A twin study from Australia that controls for genetic influences [1] confirms this.


The Australian Twin Register is a volunteer panel of twins born between 1964 and 1971. For this study they were interviewed in a single telephone interview in 1996-2000 when the median age was 30 years (range 24 to 36). There were 4010 pairs, with 861 individuals reporting cannabis use before 17 years of age and 2,911 who started using cannabis after 17 years of age.

There were 311 twins discordant in the age at which they began using cannabis, and it is these who formed the population for the study. The structured interview was adapted from an assessment of alcoholism, and included information on factors including childhood family environment.

Questions included those regarding lifetime use of drugs in a non-medical context. Those using drugs were additionally asked questions about drug abuse (use in physically hazardous situations, or interfering with major obligations) or dependence (continued use despite problems). Psychiatric disorders, early tobacco use and early regular alcohol use were also questioned.


The first (and interesting) result was that out of 6,265 individuals interviewed 3,772 (60%) reported lifetime use of cannabis. The lifetime use of drugs and of drug abuse or dependence were higher in early cannabis users than in their co-twins who reported later use (Figure 1).

Figure 1: Associations between age of first cannabis use and later use of other drugs

Any illicit drug abuse or dependence was reported by 48% of early cannabis users, compared with 33% for a later using twin. Alcohol dependence was also higher for early cannabis users (43%) than later using twins (30%). The differences were statistically significant for all classes of drug use and abuse, with the exception of abuse of sedatives.

For those who started using cannabis before they were 17 years old the odds of other drug use, alcohol dependence and other drug abuse or dependence were two to five times higher than in co-twins who did not report early cannabis use.


Any study that uses self-reporting from individuals who claim to be drug users or abusers could be criticised, but this is how much information is collected in this area. These were events occurring in the relatively recent past for these responders, and would be major determinants of their lives. Recall bias therefore seems unlikely.

The evidence is that it is the early use of cannabis, rather than any genetic predisposition that predisposes to later drug abuse and increased use of illicit drugs. Mechanisms are suggested in the paper, but it is because this study was conducted in twins that gives it authority.

Startling is the high prevalence of lifetime cannabis use, which was 60% in 6,265 people questioned. It is also possible to calculate the prevalence of any illicit drug abuse or dependence. There were 311 twins, and 250 individuals had self-reported illicit drug abuse and/or dependence, which is 4.0% of the initial sample. But this is only a sub-sample, because to be in the analysis there had to be twins discordant for early cannabis use.

In total 861 used cannabis before age 17 and 2,911 after 17 years. If the abuse and dependence for twins applied here also, then there would be 410 + 954, or 1,364 people out of 6,265 (22%) reporting illicit drug abuse or dependence in their lifetime. That is a chilling statistic.


  1. MT Lynskey et al. Escalation of drug use in early-onset cannabis users vs co-twin controls. JAMA 2003 289: 427-433.
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