Diphenhydramine (Benadryl) as it is commonly known, is an antihistamine medication that is utilized in the allergic reactions for both adults and children. As an antihistamine, Benadryl competes with histamine for cell receptor sites of effected cells (histamine H1 receptors). This reverses the effect of histamine on the capillaries resulting in reduction of allergic symptoms. Benadryl is well absorbed by the gastrointestinal tract when taken by mouth. The drug then binds to plasma allowing it to be widely distributed throughout the body. Because Diphenhydramine is taken orally, the rate at which the drug action begins is considered to be slow (30-45 minutes) with the effects of Benadryl usually last anywhere from 4-6 hours. Benadryl is metabolized by the liver resulting in Nordiphenhydramine (active metabolite), Dinordiphenhydramine, and Diphenylmethoxyacetic acid which are almost completely excreted (via urine) in 24 hours. With a rapid onset, the peak of Diphenhydramine occurs approximately 2-3 hours after administration. Diphenhydramine's half-life of 1-4 hours gives it a duration that ranges from 4-6 hours. .
Some general indications for the use of Benadryl include symptoms of allergic reaction (rash, itching, watery eyes, itchy eyes/nose/throat, sneezing, etc.), motion sickness, and Parkinson's disease. Contraindications include any hypersensitivity to antihistamines, neonatal patients, premature infants, and patients with altered LOC (when using chewable tablets). Because of Benadryl's Atropine-like action (muscle relaxant) caution should be used when administering to patients with hyperthyroidism, hypertension, increased intraocular pressure, cardiovascular disease, lower respiratory disease, narrow angle glaucoma, symptomatic prostatic hypertrophy (enlarged prostate), and bladder-neck obstruction. Diphenhydramine falls under pregnancy category B; therefore it is not expected to harm an unborn baby.