Diphenhydramine in the Management of Antipsychotic‑Induced Bradykinesia

Mechanism and Evidence Base

The American Academy of Child and Adolescent Psychiatry notes that anticholinergic agents (including antihistamines with anticholinergic activity) are established treatments for drug‑induced parkinsonism, of which bradykinesia is a core feature. 1
Antipsychotic‑induced bradykinesia represents drug‑induced parkinsonism caused by dopamine‑D₂ receptor blockade in the nigrostriatal pathway. 1
The Academy recommends anticholinergic or mild dopaminergic agents (e.g., amantadine) to treat parkinsonian symptoms—bradykinesia, tremor, rigidity—produced by antidopaminergic drugs. 1
Diphenhydramine, an antihistamine with anticholinergic properties, belongs to the medication class endorsed for treating drug‑induced parkinsonism. 1

Intravenous diphenhydramine produces symptomatic relief of antipsychotic‑induced bradykinesia within several minutes, with effects lasting approximately 4–6 hours. 2
The recommended acute‑management dose is 25–50 mg diphenhydramine administered intravenously slowly to reduce the risk of hypotension. 2

Diphenhydramine carries notable anticholinergic adverse effects—including urinary retention, dry mouth, blurred vision, constipation, and a heightened risk of delirium—particularly in older adults. 2
It should be used with extreme caution or avoided in patients who are elderly, have glaucoma, benign prostatic hyperplasia, urinary retention, dementia, or other cognitive impairments. 2
The American Geriatrics Society classifies diphenhydramine as inappropriate for older adults because it is associated with a 1.7‑fold increased risk of postoperative delirium. 2
Clinicians should monitor for paradoxical reactions (e.g., increased agitation or rage), especially in children and adolescents, which are unpredictable without prior documentation. 2
When combined with other central nervous system depressants, diphenhydramine may cause hypotension; careful hemodynamic monitoring is advised. 2

Prophylactic anticholinergic therapy may be considered for patients at high risk of acute dystonia or with a history of dystonic reactions, but the regimen should be reassessed after the acute phase. 1