What is the best course of action for a patient experiencing dystonia after taking diphenhydramine (antihistamine)?

Management of Diphenhydramine-Induced Dystonia

Immediately administer diphenhydramine 1-2 mg/kg (maximum 50 mg) IV/IM or benztropine 1-2 mg IV/IM to reverse the dystonic reaction, with benztropine preferred if the patient is allergic to diphenhydramine. 1, 2

Immediate Treatment Protocol

First-line pharmacologic intervention:

• Administer diphenhydramine 1-2 mg/kg IV/IM (maximum initial dose: 50 mg) for acute dystonic reactions 1
• Alternative: Benztropine 1-2 mg IV/IM if patient is allergic to diphenhydramine 1
• Benztropine may provide faster recovery time compared to diphenhydramine in drug-induced dystonic reactions 2
• Diazepam 5 mg IV can be used as adjunctive therapy for severe reactions 3, 4

Clinical Presentation to Recognize

Diphenhydramine-induced dystonia typically presents with:

• Rapid onset (within 2 minutes of administration) 4
• Trismus (jaw muscle spasm/lockjaw) 3, 4
• Dysarthria (difficulty speaking) 4
• Facial dystonia and buccolingual movements 4, 2
• Torticollis (neck muscle spasm) 2
• Tremors of extremities 4
• Diminished consciousness in severe cases 4
• Unilateral weakness 4

Critical Administration Warnings

Avoid rapid IV administration of diphenhydramine as treatment, as this may precipitate:

• Seizures 1
• Respiratory suppression 1
• Hypotension 1
• Paradoxical excitement 1

Post-Treatment Monitoring

• Admit for observation following treatment of the dystonic reaction 3
• Monitor for approximately 12 hours until complete resolution of symptoms 4
• Watch for residual weakness or neurological symptoms 4
• Assess mental status changes and excessive sedation from treatment medications 1

Expected Recovery Timeline

• Most patients experience complete resolution within 12 hours after appropriate treatment 4
• Symptoms typically improve rapidly after administration of anticholinergic agents 3, 4
• Hospital discharge is appropriate once symptoms fully resolve and patient is stable 3, 4

Important Clinical Pitfall

This is a paradoxical reaction: Diphenhydramine itself can cause acute dystonia, despite being a standard treatment for dystonic reactions from other medications (such as antiemetics like metoclopramide or prochlorperazine) 5, 1, 3, 4. This adverse effect occurs rarely but has been documented in multiple case reports, and can develop after both short-term and long-term therapy 4. The reaction can occur even with standard therapeutic doses (25 mg) 3, 4.