Marijuana brain scans. Coloured positron emission tomography (PET) scans of the brains of a marijuana user (bottom) and a non-marijuana user (top) showing differing levels of activity. The front of the brain is uppermost in these scans, which show progressively deeper slices from left to right. Marijuana, parts of the hemp plant (Cannabis sativa), contains the psychoactive drug tetrahydrocannabinol (THC). THC is seen to reduce brain activity in the cerebellum (lower parts of the four scans on the right). In the normal cere- bellum red areas of high activity are seen, which are absent in the marijuana user. This causes a lack of co-ordination and poor spatial judgement.