Tetrahydrocannabinol (THC) is the active chemical in cannabis and is one of the oldest hallucinogenic drugs known. There is evidence that cannabis extracts were used by the Chinese as a herbal remedy since the first century AD. Cannabis comes from the flowering tops and leaves of the hemp plant, Cannabis sativa (shown in the picture on the right).

For centuries this plant has been widely cultivated around the world for its fibres, and indeed the word canvas, which is a material made from woven hemp fibres, takes its name from cannabis. However, cannabis is more commonly known as the source of the marijuana drug, although the word marijuana applies both to the whole plant, and to the resin from it.