Sedation, drowsiness, and ataxia may occur. Paradoxical excitation is possible in small children.
Excessive doses may cause flushing, tachycardia, blurred vision, delirium, toxic psychosis, urinary retention, and respiratory depression.
Some preparations may contain sulfite preservatives, which can cause allergic-type reactions in susceptible persons.
Diphenhydramine may exacerbate dyskinetic movement disorders as a result of increased dopamine (eg, amphetamine or cocaine intoxication) or decreased cholinergic effects in the CNS.
Extravasation from an IV dose of 500 mg into arm soft tissue resulted in a chronic regional pain syndrome (case report). Local necrosis from subcutaneous route.
Use in pregnancy. FDA Category B (See Introduction in Section III). Fetal harm is extremely unlikely.