Chunk #24 — RESULTS — Genetically informative research approaches and their limitations — Comorbidity between abuse/ dependence on different substances
There is robust evidence for shared genetic influences on alcohol, nicotine and drug dependence. Two studies focusing exclusively on the genetic and environmental underpinnings of illicit drug abuse/ dependence in males reported somewhat divergent results: Kendler et al55 found no evidence for substance specific genetic influences on cannabis, sedatives, stimulants, cocaine opiates and hallucinogens. In contrast, Tsuang et al56 reported specific genetic influences on opiate abuse, although most genetic influences were shared across these substances. Expanding this work to include licit as well as illicit drug dependence in both males and females, Kendler et al57 concluded that there are two genetic factors (one predisposing largely to licit drug dependence and one to illicit drug dependence) underlying dependence on these drugs, although there was also evidence of quite large specific genetic influences on both nicotine and caffeine.
