Diphenhydramine hydrochloride is an antihistamine sold under various trade names, including Benadryl. It is administered orally, by injection and topically to the skin. The drug is available by prescription and over-the-counter. Diphenhydramine hydrochloride is indicated for allergy symptoms, motion sickness, symptoms related to Parkinson's disease, cough suppression and as a sleep aid. As part of a cream preparation, it treats itching and inflammation related to allergic skin conditions. The listed side effects refer to the drug's oral and injectable uses. Side effects of the cream preparation include skin irritation and allergic reactions.
Neurological
Common neurological side effects of diphenhydramine include disturbed coordination, fatigue, drowsiness, dizziness and sedation. Less common side effects include headache, blurred vision, double vision, restlessness, tingling skin, confusion, nervousness, tremor, inflammation of the nerves, inner ear inflammation, convulsions, excitedness and tiredness.
Cardiac and Blood-related
Cardiac and blood-related side effects are uncommon and include low heart rate, fast heart rate, extra heart beats, low blood pressure, the feeling of the heart pounding in the chest, hemolytic anemia (low level of red blood cells due to their destruction), hypoplastic anemia (low level of red blood cells due to poorly functioning bone marrow), leukopenia and agranulocytosis (low levels of white blood cells), thrombocytopenia (low platelet level) and pancytopenia (low level of all blood cells).
Respiratory
A common respiratory side effect is thickened bronchial secretions. Less common side effects include dry mouth, dry nose, dry throat, chest tightness, wheezing, stuffy nose and sore throat.
Gastrointestinal
Common gastrointestinal side effects of diphenhydramine include heartburn and vomiting. Less common side effects include nausea, loss of appetite, increased appetite, constipation and diarrhea.
Urinary and Reproductive
Urinary and reproductive side effects are uncommon and include painful urination, frequent urination, difficulty urinating, impotence, loss of sexual interest and early onset of menstruation.
Other Side Effects
A common side effect includes increased sweating. Uncommon side effects include weight gain, itching, rash, chills, sensitivity to sunlight and anaphylactic shock (shock due to allergy).
Warning
Due to the sedative effects of diphenhydramine, caution must be exercised when driving, working with machinery or performing any task that requires alertness. Avoid alcohol to decrease the risk of oversedation.
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