A large group of structurally related phenolic compounds found in the plant Cannabis sativa. The main one is tetrahydrocannabinol (THC), which is the compound responsible for the effects of cannabis. It effects cannabinoid receptors found in the brain (and also in the spleen). The name ‘cannabinoid’ is also applied to structurally unrelated compounds found naturally in animal tissue and having an affect on the cannabinoid receptors. These endocannabinoids (endogenous cannabinoids) are believed to act as ‘messengers’ between cells.