Formication is a specific form of the general set of abnormal skin sensations known as paresthesia, and thus it is related to the sensation known as "pins and needles", and other tingling sensations.
Some people suffering the sensation of formication find it to be annoying, others find it painful, and some find it itchy. Those who find it to be itchy may in some cases repeatedly scratch themselves until they bleed, causing skin damage and sores. (In the subset of cases where the sufferer is delirious or intoxicated because of high fever, substance abuse, or extreme alcohol withdrawal, this repeated scratching is very common indeed.)
Formication can on occasion lead to people becoming fixated on the sensation and its possible meaning, and these people may develop delusional parasitosis. This is a situation where individuals are convinced that there are real insects crawling on and/or under their skin, whereas in reality there are no insects involved, just a crawling sensation. It is fairly easy to misunderstand the significance and causality of the "creepy crawly" sensation of formication.
In addition, formication can be caused by the withdrawal component of substance abuse. It is a common side-effect of the extensive use of cocaine or methamphetamine, or the abusive use of amphetamines. Extreme alcohol withdrawal may also cause symptoms of formication, along with delirium tremens, and can often be accompanied by visual hallucinations of insects.
A variety of itching, often encountered in the eczema of elderly people, is formication; this is described as exactly like the crawling of myriads of animals over the skin. It is probably due to the successive irritation of nerve fibrils in the skin. At times patients who suffer from it will scarcely be persuaded that it is not due to insects. Yielding to the temptation to scratch invariably makes the disease worse.
...a formication, or sensation as of ants or other small insects creeping on the parts.
In the movie Old Boy, the main character experiences formication while imprisoned for 15 years.
The word formication appears in the epic David Foster Wallace's novel, Infinite Jest, when it was used in a character's description of DTs in alcohol detox.
In the movie Friday (film), Smokey (Friday) has a flashback showing him smoking a joint, while his friends tell him to take it easy. They then tell him it was angel dust. Soon Smokey experiences formication, and starts scratching his skin.