 This trial will examine the efficacy of lithium in providing symptomatic relief from the withdrawal discomfort experienced by some dependent users of cannabis on cessation of regular use. Significant withdrawal may be a barrier to achieving abstinence in some clients and can be associated with marked disturbances in mood, sleep, hostility and aggression. Relief from such symptoms may be important in helping some clients achieve a period of abstinence and facilitate subsequent entry into a relapse prevention program. 