This study examines third-person perceptions (TPP) of two types of online messages--antisocial messages that encourage drug abuse and prosocial messages in the youth anti-drug campaign--and their relationship with support for three types of rectifying measures: restrictive, corrective, and promotional. A survey of 778 secondary school students (approximately equivalent to students in Grades 7-13) in Hong Kong found that the self-other perceptual gap in effects of prosocial messages was significantly smaller than that of antisocial messages. Regression analysis showed that the perceptual gap of antisocial messages significantly predicted greater support for all three types of rectifying measures, but the perceptual gap of prosocial messages did not. More emphasis should be laid on educating students about the possible harmful effects of online drug-encouraging messages among their peers, which might help promote support for restrictive, corrective, and promotional anti-drug measures.