If symptoms persist or are severe, a dose reduction or switch to another antihistamine may be advisable. Other possible CNS effects include headache and hallucinations. Antihistamines sometimes can cause CNS stimulation. This effect is more likely to occur in children with overdoses, and symptoms may include restlessness or akathisia, insomnia, palpitations, and seizures. Diphenhydramine possesses a moderate degree of anticholinergic actions.
Geriatric patients are more likely to experience such adverse reactions.
Some of these adverse reactions may be relieved by taking the drug with meals or milk. Adverse cardiovascular responses are likely to be associated with the anticholinergic properties or the quinidine-like anesthetic effects of antihistamines. Alpha-adrenergic blockade can lead to symptoms of hypotension. Hypertension can also occur. Topically application of diphenhydramine can result in a hypersensitivity reaction.