Unusual treatment response of a severe dystonia to diphenhydramine

A 27-year-old man was admitted to the emergency department with a fluphenazine decanoate-induced dystonia. He was treated with 125 mg diphenhydramine IV in four doses and 2 mg benztropine IM. A fluctuating response was observed before continued remission of the dystonia. Possible reasons for variable patient responses to diphenhydramine are discussed.