Alcohol withdrawal. Coloured positron emission tomography (PET) scans of horizontal slices of an alcoholic's brain during withdrawal. The front of the brain is at top, and the slices get deeper from left to right. The brighter (yellow) brain areas are those of high activity. The activity of the brain after, from top to bottom, ten days, 21 days and 30 days is seen. Brain activity is seen to increase with increasing time without alcohol. Alcohol is a depressant, and is addictive if taken often in large doses. In PET scans, radioactively- tagged molecules are injected into the bloodstream and their emissions monitored. Glucose is used here, as it is taken up by active brain cells.