Formication is a tactile hallucination that insects or snakes are crawling over or under the skin.

Formication is a common side-effect of extensive use of cocaine or amphetamines, it can also be experienced with high fevers. Extreme alcohol withdrawal may also cause symptoms of formication, often associated with visual hallucinations of insects.

People suffering from this hallucination may scratch themselves to the extent of serious skin damage and bleeding, especially if they are delirious or intoxicated.