Benadryl

Benadryl is a brand name used for products that contain the antihistamine diphenhydramine. Diphenhydramine is a first‑generation H1 antihistamine that blocks the effects of histamine and is used to relieve symptoms of allergies and hay fever, such as sneezing, runny nose, itching, and itchy, watery eyes. It is also used to treat hives and, in some markets, to relieve itching due to insect bites or rashes, and in certain forms to aid sleep.

Benadryl is sold in several dosage forms including tablets, caplets, chewable tablets, and liquid preparations for adults and children. Some products combine diphenhydramine with other ingredients such as decongestants to relieve cold symptoms. In some countries, injectable forms are used in clinical settings.

Diphenhydramine crosses the blood–brain barrier, producing sedative effects in addition to allergy relief. Onset of action is typically within 15 to 60 minutes when taken orally, and the effects for allergy relief generally last several hours; sedation may persist longer, especially in older people.

Common adverse effects include drowsiness, dry mouth, dizziness, blurred vision, and urinary retention. Because of its anticholinergic effects, it can cause confusion or constipation in older adults and may worsen glaucoma or urinary retention. It should be used with caution in people who drink alcohol or who operate machinery. It is usually not recommended for infants under two years in many jurisdictions and should be used in pregnancy only if clearly needed and directed by a clinician; nursing mothers should consult a health professional before use.

Diphenhydramine can interact with alcohol, sedatives, and certain antidepressants and anti-Parkinson medications, increasing CNS depression or anticholinergic effects. Always follow the product label or a clinician’s guidance for dosing, and seek medical attention in cases of suspected overdose, which may include severe drowsiness, agitation, seizures, or coma.