This study employed the theory of planned behavior (TPB) to investigate the factors underlying intentions to use and use of LSD, amphetamine, cannabis, and ecstasy over 6 months in a sample of students (N= 461). The TPB provided good predictions of both intentions (mean R2= .49) and behavior (mean R2= .45). Descriptive norms explained additional variance in intentions for all the drugs, and moral norms explained additional variance in cannabis intentions. Attitude variability moderated the impact of attitudes on intentions for LSD (p < .10) and ecstasy (p < .05). Attitude moderated the impact of perceived behavioral control (PBC) on intentions for all drugs (p < .001). PBC moderated the impact of intentions on behavior for LSD (p < .05), amphetamine (p < .10), cannabis (p < .05), and ecstasy (p < .10). These interaction effects elucidate limiting conditions among the variables in the TPB.