The present case-control study shows that DRD2 contributes to the variation in the risk for heroin dependence in Spanish individuals. The A1-A1 genotype was significantly associated with heroin dependence in the entire sample, while the A1 allele was associated only in males. Accordingly, the conclusion of a series of meta-analyses is that DRD2 TaqI A polymorphism is associated with SUD, particularly with severe SUD [16]. We consider that the patients in the present study suffered a severe SUD, because they had a long history (≈10 years) of mostly IV heroin use, which was frequently combined with non-opioid substance use.
