Overdose symptoms may include:

Acute poisoning can be fatal, leading to cardiovascular collapse and death in 2–18 hours, and in general is treated using a symptomatic and supportive approach. Diagnosis of toxicity is based on history and clinical presentation, and in general specific levels are not useful. Several levels of evidence strongly indicate diphenhydramine (similar to chlorpheniramine) can block the delayed rectifier potassium channel and, as a consequence, prolong the QT interval, leading to cardiac arrhythmias such as torsades de pointes. No specific antidote for diphenhydramine toxicity is known, but the anticholinergic syndrome has been treated with physostigmine for severe delirium or tachycardia. Benzodiazepines may be administered to decrease the likelihood of psychosis, agitation, and seizures in people who are prone to these symptoms. Interactions
Alcohol may increase the drowsiness caused by diphenhydramine. Pharmacology
Pharmacodynamics
Diphenhydramine, while traditionally known as an antagonist, acts primarily as an inverse agonist of the histamine H1 receptor. It is a member of the ethanolamine class of antihistaminergic agents. By reversing the effects of histamine on the capillaries, it can reduce the intensity of allergic symptoms. It also crosses the blood–brain barrier and inversely agonizes the H1 receptors centrally. Its effects on central H1 receptors cause drowsiness. Like many other first-generation antihistamines, diphenhydramine is also a potent antimuscarinic (a competitive antagonist of muscarinic acetylcholine receptors) and, as such, at high doses can cause anticholinergic syndrome. The utility of diphenhydramine as an antiparkinson agent is the result of its blocking properties on the muscarinic acetylcholine receptors in the brain.
Other side effects include motor impairment (ataxia), flushed skin, blurred vision at nearpoint owing to lack of accommodation (cycloplegia), abnormal sensitivity to bright light (photophobia), sedation, difficulty concentrating, short-term memory loss, visual disturbances, irregular breathing, dizziness, irritability, itchy skin, confusion, increased body temperature (in general, in the hands and/or feet), temporary erectile dysfunction, and excitability, and although it can be used to treat nausea, higher doses may cause vomiting. Diphenhydramine in overdose may occasionally result in QT prolongation.Some individuals experience an allergic reaction to diphenhydramine in the form of hives.Conditions such as restlessness or akathisia can worsen from increased levels of diphenhydramine, especially with recreational dosages. Normal doses of diphenhydramine, like other first generation antihistamines, can also make symptoms of restless legs syndrome worse. As diphenhydramine is extensively metabolized by the liver, caution should be exercised when giving the drug to individuals with hepatic impairment. Anticholinergic use later in life is associated with an increased risk for cognitive decline and dementia among older people. Contraindications
Diphenhydramine is contraindicated in premature infants and neonates as well as people who are breastfeeding. It is a pregnancy Category B drug. Diphenhydramine has additive effects with alcohol and other CNS depressants. Monoamine Oxidase inhibitors prolong and intensify the anticholinergic effect of antihistamines. Overdose
Diphenhydramine is one of the most commonly misused over-the-counter drugs in the United States. In cases of extreme overdose, if not treated in time, acute diphenhydramine poisoning may have serious and potentially fatal consequences.