Diphenhydramine overdosage in adults causes CNS depression with drowsiness or coma which may be followed by excitement, seizures, and postictal depression. Impaired consciousness, psychosis, tachycardia and mydriasis have also been reported.
The following signs and symptoms of overdosage have been reported in children: psychosis, fixed dilated pupils, abnormal eye movements, flushed face, dry mouth, urinary retention, fever, excitation, hallucinations/delusions, disorientation, agitation, confusion, jitteriness, restlessness, irritability, hyperactivity, delirium, twitching, tiredness, abnormal tongue movement, unsteady pattern of walking, trembling extremities, slurred speech, ataxia, athetosis, seizures, and postictal depression.
Treatment should be supportive and directed towards specific symptoms.
If vomiting has not occurred spontaneously the patient should be induced to vomit. This is best done by having him drink a glass of water or milk after which he should be made to gag. Precautions against aspiration must be taken, especially in infants and children.
If vomiting is unsuccessful, gastric lavage is indicated within three hours after ingestion and even later if large amounts of milk or cream were given beforehand. Isotonic or (1/2) isotonic saline is the lavage solution of choice.
Saline cathartics, such as milk of magnesia, draw water into the bowel by osmosis and therefore are valuable for their action in rapid dilution of bowel content.
Stimulants should not be used.
Vasopressors may be used to treat hypotension. Convulsions and marked CNS stimulation should be treated with parenteral diazepam.