Pharmacy Author: Omudhome Ogbru, PharmD
Medical and Pharmacy Editor: Jay W. Marks, MD

GENERIC NAME: diphenhydramine

BRAND NAME: Benadryl

DRUG CLASS AND MECHANISM: Diphenhydramine is an antihistamine used for treating allergic reactions. Histamine is released by the body during several types of allergic reactions and--to a lesser extent--during some viral infections, such as the common cold. When histamine binds to its receptors on cells, it stimulates changes within the cells that lead to sneezing, itching, and increased mucus production. Antihistamines compete with histamine for cell receptors; however, when they bind to the receptors they do not stimulate the cells. In addition, they prevent histamine from binding and stimulating the cells. Diphenhydramine also blocks the action of acetylcholine (anticholinergic effect) and is used as a sedative because it causes drowsiness. The FDA originally approved diphenhydramine in 1946.

GENERIC AVAILABLE: Yes

PRESCRIPTION: No

PREPARATIONS: Capsules: 25 and 50 mg. Tablets: 12.5, 25, and 50 mg. Strips: 12.5 and 25 mg. Elixir, oral solution, liquid: 12.5 mg per teaspoon (5 mL). Suspension: 25 mg per 5 ml. Injection: 50 mg per ml

b Diphenhydramine should be stored at room temperature, 15° to 30°C (59° to 86°F), and protected injection from freezing and light.

PRESCRIBED FOR: Diphenhydramine is used for the relief of nasal and non-nasal symptoms of various allergic conditions such as seasonal allergic rhinitis. It is also used to alleviate cold symptoms and chronic urticaria (hives). Although antihistamines are the preferred class of drugs in allergic rhinitis, they only reduce symptoms by 40%-60%. Diphenhydramine also is used for allergic reactions involving the eyes (allergic conjunctivitis), to prevent or treat active motion sickness, and for mild cases of Parkinsonism, including drug-induced Parkinsonism. The last two uses (motion sickness and Parkinsonism) are based on the anticholinergic effects of diphenhydramine, and not its antihistamine effects. Diphenhydramine is also used for treating insomnia.

DOSING: Diphenhydramine has its maximal effect about one hour after it is taken. When used to combat insomnia, it is prescribed at bedtime. Patients over the age of 60 years are especially sensitive to the sedating and anticholinergic effects of diphenhydramine, and the dose should be reduced. Doses vary depending on formulation. A common regimen for treating adult allergic reaction is 25-50 mg every 4-6 hours not to exceed 300 mg daily.

DRUG INTERACTIONS: Diphenhydramine adds to (exaggerates) the sedating effects of alcohol and other drugs than can cause sedation such as the benzodiazepine class of anti–anxiety drugs [for example, diazepam (Valium), lorazepam (Ativan), clonazepam (Klonopin), alprazolam (Xanax)], the narcotic class of pain medications and its derivatives [for example, oxycodone and acetaminophen (Percocet), and hydrocodone and acetaminophen (Vicodin), guaifenesin with hydromorphone (Dilaudid), Codeine, propoxyphene (Darvon)], the tricyclic class of antidepressants [for example, amitriptyline (Elavil), imipramine (Tofranil), desipramine (Norpramin)], and certain antihypertensive medications [for example, clonidine (Catapres), propranolol (Inderal)]. Diphenhydramine can also intensify the drying effects of other medications with anticholinergic properties [for example, dicyclomine (Bentyl), bethanechol (Urecholine), Probanthine].

PREGNANCY: Diphenhydramine has not been adequately evaluated in pregnant women. It should be used only if the benefit outweighs the potential but unknown risks.

NURSING MOTHERS: Diphenhydramine is secreted in breast milk. Because of the risk of stimulation and seizures in infants, especially newborns and premature infants, antihistamines should not be used by nursing mothers.

SIDE EFFECTS: Diphenhydramine can commonly cause sedation, tiredness, sleepiness, dizziness, disturbed coordination, drying and thickening of oral and other respiratory secretions, and stomach distress. Diphenhydramine may also cause low blood pressure, palpitations, increased heart rate, confusion, nervousness, irritability, blurred vision, double vision, tremor, loss or appetite, or nausea. Diphenhydramine should be used with caution (if at all) in persons with narrow-angle glaucoma, prostatic hypertrophy (enlarged prostate gland), hyperthyroidism, cardiovascular disease, hypertension, and asthma.

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