The dopamine transporter has also been studied in relation to stimulant drug abuse. In these studies, the exact doses of drug are unknown and presumed to be high relative to laboratory or clinical studies. A case–control study examined methamphetamine dependence and drug-induced psychosis in a cohort of individuals with methamphetamine use disorder and psychosis (Ujike et al. 2003). No associations were found between the four polymorphisms tested and methamphetamine dependence or psychosis. However, when the patients with prolonged psychosis were analyzed separately from those with transient psychosis, there was a strong association indicating that individuals with nine or fewer repeat alleles of the 3′-UTR VNTR had prolonged psychosis. The 9-repeat allele was also marginally associated with increased risk of cocaine-induced paranoia in cocaine dependent subjects when the frequency of the 9-repeat allele was compared in individuals with and without psychosis (Gelernter et al. 1994). The reasons for these associations are unclear, given the lack of information about the doses that were ingested. The findings may indicate that drug users with the 9-repeat allele of the 3′-UTR VNTR polymorphism are more
