What happens when a friend starts talking about her own substance use and misuse? This article provides the first investigation of how substance use is spontaneously topicalized in naturally occurring conversation. It presents a detailed analysis of a rare video-recorded interaction showing American English-speaking university students talking about their own substance (mis)use in a residential setting. During this conversation, several substance (mis)use informings are disclosed about one participant, and this study elucidates what occasions each disclosure, and how participants respond to each disclosure. This research shows how participants use casual conversation to offer important substance (mis)use information to their friends and cohabitants, tacitly recruiting their surveillance. Analysis also uncovers how an emerging adult peer group enacts informal social control, locally (re-)constituting taken-for-granted social norms and the participants' social relationships, to on the one hand promote alcohol use while, on the other hand endeavouring to prevent one member from engaging in continued pain medication misuse. This article thus illuminates ordinary peer conversation as an important site for continued sociological research on substance (mis)use and prevention.