One of the markers investigated in our study, rs1803274, was not associated with cocaine dependence but was associated with crack smoking as the preferred route of administration. Although the rs1803274 marker has previously been shown to be associated with behavioral and medical conditions [33], [34], we found no studies investigating the role of rs1803274 in substance dependence or other substance-related psychiatric disorders to date. The fact that we found an association only when the sample was divided into subgroups is not unique. An association with a genetic marker in cocaine dependence was found when the sample was subdivided into those who had experienced psychotic symptoms during episodes of cocaine intoxication and those who had not [35]. Because cocaine dependent subjects constitute a heterogeneous group not only from a genetic standpoint but also from a clinical perspective, it is useful to study them in meaningful subgroups. Dual users have distinct clinical features that distinguish them from those who are exclusive users of crack or snorted cocaine [13]. In the present study, allele frequencies for rs1803274 were very similar to the values
