Analysis of the MWC revealed a significant main effect for trial (F(1, 37)=7.95, p<0.01), indicating that withdrawal symptom severity increased after abstinence. A 2 (Trial) × 2 (Genotype) mixed ANOVA revealed a significant Genotype × Trial interaction (F(1, 37)= 4.99, p=0.03), such that C/C individuals reported more severe withdrawal symptoms than A/A and A/C individuals (see Fig. 1). Because the genotype groups differed with respect to this variable, it was co-varied in all subsequent genotype analyses, to account for the possibility that changes in craving or sensitivity to acute effects were caused by differences in withdrawal symptom severity.
