This article investigates self-reported interventions by students in the alcohol, tobacco, illicit drug use, and drinking-driving of their friends. The data came from a study of 1184 students in Ontario schools in grades 7, 9, 11, and 13. We found that about a third of students intervened in friends' illegal drug use and drinking-driving but about half intervened about smoking. Students who intervened were more likely to be older and spend fewer nights at home. They were less likely to use cannabis, but had more friends using cannabis and illegal drugs. Also, they had more exposure to drug education and were more disapproving of drug use. Drug education may give students the knowledge and confidence to intervene in friends' drug use.