Recent work contributing towards an understanding of the mechanism of action of cannabis and its constituents is described and the known therapeutic and pharmacological activities of these substances reviewed, together with synthetic drugs derived from tetrahydrocannabinol (A'-THC). The medical uses discussed include the actions of cannabinoids in the treatment of glaucoma, asthma, emesis, hyperthermia, convulsion, muscle spasticity, anxiety, hypertension, pain and inflammation.