The author presents a brief history of the cultivation and use of cannabis and explains the background to attempts to isolate the active ingredients of the drug. Short term and long term effects are briefly described and the long term behavioural changes, which are of particular concern to doctors in general practice are examined. The role of the doctor in dealing with acute reactions to cannabis use is outlined, and the possibility of recurring anxiety, more commonly associated with use of LSD, is identified as a possible cause of concern.