Self-medication theory (SMT) posits that individuals exposed to trauma and resulting posttraumatic stress symptoms (PTSD) are at risk for heavy drinking and associated negative consequences. Close peer alcohol use is also a powerful predictor of alcohol involvement in college, particularly influencing those with greater negative affect. As individuals with PTSD may rely on peers for support, peer drinking behaviors are possibly putting them at further risk for greater alcohol use and resulting consequences. To test self-medication processes, the present study examined the relationship between weekday PTSD symptoms, weekend alcohol behavior, and the influence of both emotionally supportive peer and other friend drinking behavior by investigating: (a) whether weekday PTSD symptoms predicted subsequent weekend alcohol use and consequences; and (b) whether the relationship between weekday PTSD symptoms and weekend alcohol behavior was moderated by various drinking behaviors of one’s peers. Trauma-exposed heavy-drinking college students (N = 128) completed a baseline assessment and 30 daily, Web-based assessments of alcohol use and related consequences, PTSD symptoms, and peer alcohol behavior. Results directly testing SMT were not supported. However, friend alcohol behavior moderated the relationship between weekday PTSD and weekend alcohol behavior. Findings highlight the importance of peer drinking as both a buffer and risk factor for problematic drinking and provide useful information for interventions aimed at high-risk drinkers.