association studies. Another confounding factor in previous studies was a concomitant dependence on other drugs, mainly alcohol and heroin. In the present study, individuals with alcohol dependence were excluded, and heroin dependence is very rare in Brazil (less than 1% of our sample used heroin). This attempt to make the sample more homogeneous may strengthen the specificity of any genetic association found as related to the condition of cocaine use per se. From a statistical standpoint, our sample had a power of 88% of detecting true associations.
