Using the discordant twin method, Lynskey and colleagues (36) selected pairs of MZ and DZ twins where one twin had used cannabis prior to age 17 and the other had used it later or not at all. They found that regardless of zygosity, individuals who used cannabis prior to age 17, when compared to their MZ or DZ cotwin (who was a never or late-onset user) were more likely to report use and misuse of harder drugs as well as cannabis use disorders. Even after adjustment for covariates such as tobacco and early alcohol use, conduct disorder, major depression and social anxiety, early onset cannabis users were at 2.4 to 3.9 increased odds of use of drugs like cocaine, hallucinogens, sedatives and opioids and twice as likely as their co-twins to meet criteria for dependence on hard drugs and alcohol. This excess risk in MZ twins indicated that causal explanations could not be excluded.
