One route to influence in mass communication campaigns to reduce risky behavior is through interpersonal discussion of the content of the campaign and other behaviors pertinent to those targeted by the campaign. The goal of this study was to test the effects of online group interaction among adolescents about antimarijuana advertisements on relevant attitudes and behaviors. A between-subjects post-only experimental design was used to test two crossed factors, online chat and strength of arguments in antidrug ads. A sample of 535 students was randomly assigned to one of four conditions: chat and strong-argument ads, chat and weak-argument ads, no chat and strong-argument ads, and no chat and weak-argument ads. The group interactions about antidrug ads lead to negative effects such that those who chatted reported more promarijuana attitudes and subjective normative beliefs than those who just viewed the ads. No support was found for the hypothesis that strong-argument ads would result in more antidrug beliefs relative to weak-argument ads in either the chat or the no-chat conditions. Overall, these findings suggest that viewing antidrug ads and discussing them with peers may result in deleterious effects in adolescents.