Clinical studies of a variety of cannabis constituents and THC isomers, homologs and metabolites have elucidated some structure-activity relationships. The fundamental structure of THC is required for pharmacological activity, but potency may be altered greatly among different double-bond isomers, by changing length of side-chain or by metabolic hydroxylations. No material has been found in nature, either in cannabis itself or in the metabolites of THC, which differs qualitatively from THC.