Research has shown significant declines in gateway drug use among participants in a school/community substance abuse prevention intervention in a midwestern, suburban school district (Lohrmann, Alter, Greene, & Younoszai, 2005). Though still at or below national levels, student marijuana use was not impacted as positively. The current study investigated the possibility that efforts to prevent alcohol use resulted in an unintentional substitution effect thereby increasing marijuana use. Factors including perceived access to alcohol and marijuana, along with perceived harm associated with alcohol and marijuana use, were examined to determine their role in marijuana use. Findings revealed a relationship between perceived access to and perceptions of harm associated with marijuana and its use that depended on the level of perceived access to and harm associated with alcohol. (Contains 1 figure, 1 footnote and 5 tables.)