Hypersensitivity to diphenhydramine HCl and other antihistamines of similar chemical structure. Because of the risk of local necrosis, this drug in the parenteral from should not be used as a local anesthetic. In infants and children, especially, antihistamines in overdosage may cause hallucinations, convulsions, or death. As in adults, antihistamines may diminish mental alertness in children. In the young child, particularly, they may produce excitation.
Antihistamines are more likely to cause dizziness, sedation, and hypotension in elderly patients.
Information for the Patient Patients taking diphenhydramine HCl should be advised that this drug may cause drowsiness and has an additive effect with alcohol. Patients should be warned about engaging in activities requiring mental alertness such as driving a car or operating appliances, machinery, etc. Carcinogenesis, Mutagenesis, and Impairment of Fertility Long-term studies in animals to determine mutagenic and carcinogenic potential have not been performed. Pregnancy Category B Reproduction studies have been performed in rats and rabbits at doses up to 5 times the human dose and have revealed no evidence of impaired fertility or harm to the fetus due to diphenhydramine HCl. There are, however, no adequate and well-controlled studies in pregnant women.