Very few patients die from an antihistamine overdose unless the dose causes serious heart rhythm disturbances or breathing problems, according to the U.S. National Library of Medicine. Diphenhydramine is an antihistamine that is used to relieve allergy symptoms.
An overdose can occur anytime a person takes more than the recommended dosage. Some symptoms of overdose include agitation, convulsions, nausea, rapid heart rate, blurred vision, coma and stomach pain. Never hesitate to call 911 if an individual has overdosed on any medication. If a person is suffering from an overdose, treatment at the hospital will include activated charcoal, breathing support, intravenous fluids and gastric lavage.