A 45-year-old woman was administered oral and intravenous diphenhydramine 25 mg for the treatment of an allergic reaction. Within 2 minutes she rapidly developed trismus, dysarthria, tremors of the upper extremities, left-sided weakness, and diminished consciousness. She was treated with intravenous diazepam and benztropine with good response. After approximately 12 hours the patient's condition was completely resolved except for minor subjective weakness of her left extremities. Her hospital stay was uneventful, and she was discharged after 4 days after refusing rechallenge with the drug. Several cases of acute dystonic reactions secondary to antihistamines have been reported in the literature, four of which involved diphenhydramine. Such reactions may occur after short- or long-term therapy. Most patients experienced rapidly developing trismus, facial dystonia, dysarthria, and occasionally, decreases in consciousness, motor incoordination, and weakness. Because of the widespread availability of diphenhydramine and other antihistamines to the general public, awareness of this effect is of great importance.