QWhich medication is best for patients with delusions of parasitosis?

Delusions of parasitosis (DOP) is a condition that is not uncommonly seen in any busy dermatology practice. Patients present with the fixed, false belief that they are either infested with parasites or experience inert material, such as fibers, coming out of their skin. Unfortunately, these patients present to the dermatologist because they believe they have a bona fide skin condition (but in reality, this is mostly a psychiatric condition). Given this belief regarding a bona fide skin condition, these patients are extremely averse to any inkling of mental health or psychiatry and will often vehemently refuse to see a psychiatrist. Therefore, the dermatologist must be comfortable managing these patients.

Luckily, there is one medication (pimozide), which works very well for the treatment of delusions of parasitosis. Not only is pimozide efficacious, but it is uniquely acceptable to these patients due to the fact it has no official U.S.  Food and Drug Administration (FDA) psychiatric indication: it is only indicated in the U.S. for Tourette’s syndrome, which is a neurological disorder. DOP patients are usually willing to try pimozide for this reason. Sometimes, the pharmacist will tell the patient that pimozide is a psychiatric medication. In these situations, the patient can simply be told the pharmacist is wrong and to refer to the package insert. When used in the dermatological setting (as opposed to treatment of psychiatric illnesses such as schizophrenia), the side effects are mild and not serious when used with the low doses needed to treat the condition. After nearly 50 years of worldwide use of pimozide in dermatology, there has been hardly any cases of tardive dyskinesia (a movement disorder that causes a range of repetitive muscle movements in the face, neck, arms and legs that can be permanent). If the patient happens to develop tardive dyskinesia, it is good for dermatologists to be aware that valbenazine and deutetrabenazine were approved for treatment of tardive dyskinesia in 2017. However, this is extremely rare given the low doses and short course used when treating DOP. 

References:

1. Brownstone N, Koo J. Why is pimozide uniquely useful for the treatment of delusional patients in dermatology? J Dermatol Treat. 2022;33(3):1779-1779. doi:10.1080/09546634.2020.1825611