Coloured 3D magnetic resonance imaging (MRI) scan of a brain affected by chronic methamphetamine abuse. There is a reduction in grey matter in the anterior cingulate cortex (orange), an area of the brain involved in complex cognitive functions including impulse control, decision making and empathy. There is also a reduction in the size of the hippocampus (yellow), which is responsible for long-term memory and an increase in white matter in the temporal lobe (green), which is involved in auditory processing, speech and language functions. Methamphetamine is a stimulant and euphoriant drug that is illegal in most countries. Frequent use of the drug can lead to addiction and dependence.