For diagnoses of individuals selected due to dependence on a specific substance, eg “methamphetamine dependence”, the specificity of effects of a gene’s variants is likely to be limited by the fact that many of the subjects for these studies also report use of additional addictive substances (eg inhalants for the methamphetamine dependence Samples 4 and 5). These clinical considerations, as well as the overlap between the “methamphetamine dependence” genes and the genes identified in other genome wide association work, support the idea that many, but not all, of the methamphetamine dependence (or nicotine dependence) loci are likely to contain allelic variants that provide a more general vulnerability to addictive substances. When we term some genes “methamphetamine dependence” genes, for example, to denote the fact that variants in these genes are likely to alter vulnerability to developing dependence on this substance, it is likely that many of these allelic variants also predispose individuals to dependence on other addictive substances.
