Many people have at some time experienced the eerie sensation of insects or other tiny pest (including fungi) crawling or growing on or under their skin. This is usually quite harmless though extremely irritating but in a few cases problems escalate as the person starts to scratch excessively, damaging their skin and exposing themselves to infection or longer term skin damage. This can even turn into a more damaging anxiety disorder.
It is of course possible that the person has an infestation or infection so firstly a quick visit to their doctor for examination and possibly tests usually resolves the issue. However in a few cases it does not, or the person concerns does not believe their doctors conclusion - the medical term is formication and reports date back to 1889.
There are many possibilities to explain this sensation, some of which may not have occured to you or even your doctor. Some drugs (recreational (including alchohol) as well as prescription) are known to cause this sensation as a side effect or even after stopping taking the drug! In a few rare cases when all else has been ruled out the person has to seek behavioural treatment for their strength of belief that something that is happening is not.
I think we can conclude that this is a potentially severe illness of the type that is very difficult to manage, having physical as well as psycological contributors to the cause. In the worst cases the patient is so convinced that they are right and that there is something crawling on their skin that it is impossible to convince them otherwise - which must be an awful situation for everyone concerned. Hopefully this article and others like it can raise awareness of this disorder for the good of all.
Skin crawling, causes e.g. menopause, diabetes, mercury poisoning, Lyme disease, Herpes zoster and include recreational drug abuse, alcohol abuse and lots more.
Formication reports for people taking antifungal drugs
Formication reports for people taking steroid medication
Delusions of Parasitosis Clinical Presentation An article written more tfor the medical professional and talks a lot about delusion but worth reading for the list of conditions that have been reported to mimick 'skin crawling' including dementia, treatment of Parkinsons disorder using gabapentin, severe depression.