Formication

Formication is a somewhat unusual, but medically well-known, abnormal sensation. This sensation closely resembles the feeling of insects crawling on and/or under the skin, and can also include sensations which resemble those of insects stinging or biting. There are many known causes of formication.

The word is derived etymologically from the Latin word formica, meaning "ant", precisely because of this similarity in sensation to that of crawling insects.

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.

Causes
Causes of formication include medical conditions such as diabetic neuropathy, skin cancer, or herpes zoster as well as normal states such as menopause (i.e. hormone withdrawal). Formication can also sometimes be experienced during high fevers. Itching, tingling and formication ("the creeps") often occur when surfacing from a dive or during ascent to altitude (decompression sickness).

It can be a rare side effect of many prescription drugs such as Ritalin, Adderall and Lunesta.

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.

Referenced
Formication was neatly described in 1890: 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.

The term formication has been in use for several hundred years. In the 1797 edition of the Encyclopædia Britannica, a description of the condition raphania includes the symptom: "...a formication, or sensation as of ants or other small insects creeping on the parts."

Depictions of formication in fiction

 * The novel A Scanner Darkly by Philip K Dick contains a scene where the character Jerry Fabin (and Charles Freck in the movie of the same name) suffers from formication and attempts to remedy the condition by showering for hours on end.
 * In the movie Old Boy, the main character experiences formication while imprisoned for 15 years.