Forms of cannabis and cocaine: a twin study

Cannabis and cocaine are illicit psychoactive substances that have fallen under intense scrutiny by epidemiologists and behavioral geneticists. However, most analyses have used a composite variable to represent the use of these two drugs. For example, the composite variable of cannabis use often includes use of marijuana or hashish. Similarly, cocaine use involves different preparations (crack vs. cocaine hydrochloride) and varying routes of administration (intranasal insufflation vs. smoking). While there is some epidemiological evidence for the difference in addictive potentials between crack and intranasal cocaine, genetically informative studies have not examined the relationship between the forms of cannabis or cocaine. We used data from male and female same-sex twin pairs to examine the extent of genetic, shared environmental, and unique environmental overlap between (i) marijuana and hashish for cannabis use and (ii) intranasal and crack cocaine for cocaine use. Bivariate Cholesky models were fit using the structural equation modeling software Mx. Our results indicate that for both drugs, the individual drug forms show a complete overlap of genetic factors and a substantial overlap of shared environmental influences. While marijuana and hashish share a moderate proportion of their unique environment, crack and intranasal cocaine only show a modest overlap of unique environmental factors, adding some evidence for form-specific environmental factors. In conclusion, there is substantial overlap of familial factors between forms of a single drug and preference is primarily determined by unique environmental influences. These findings also reinforce the validity of composite variables in epidemiological and genetic research.