This paper examines the believability of strong warnings about the negative consequences of drug use among young adults in Australia who have never tried, currently use, or have tried and rejected cannabis. It finds that the strong warnings about cannabis are generally believed by never-triers. The same warnings are perceived by current users as only slightly believable. Surprisingly, but also consistent with cognitive dissonance, trier-rejecters of cannabis are the most likely, more so even than never-triers, to believe the warnings about cannabis. The paper also examines warnings about cocaine and heroin by cannabis usage status. Current users of cannabis, compared with non-users, perceive the warnings about the harmful effects of cocaine as less believable, suggesting possible "gateway" susceptibility to trial of this drug. But for heroin, all groups perceive the warnings very believable. The beliefs about particular negative consequences that are most likely to lead to discontinuation of use of cannabis, and those that should discourage uptake of cocaine and heroin, are identified. (Contains 3 tables.)