diphenhydramine, Benadryl

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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 receptors on cells, it stimulates changes within the cells that lead to the release of chemicals that cause sneezing, itching, and increased mucus production. Antihistamines compete with histamine for cell receptors and bind to the receptors without stimulating 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. Yes (Injection)

PREPARATIONS:

  • Capsules: 25 and 50 mg. Tablets: 12.5, 25 and 50 mg.
  • Tablet (Chewable or dispersible)): 12.5 mg.
  • Strips: 12.5 and 25 mg.
  • Elixir, oral solution, liquid: 12.5 mg per teaspoon (5 mL).
  • Strip: 25 mg. Injection: 50 mg per ml.

STORAGE: Diphenhydramine should be stored at room temperature, 15 C to 30 C (59 F to 86 F), and the injection should be protected 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 (hay fever). It also is used to alleviate cold symptoms and hives (chronic urticaria). Although antihistamines are the preferred class of drugs in allergic rhinitis, they only reduce symptoms by 40% to 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 also is used for treating insomnia.

DOSING: Diphenhydramine doses vary depending on formulation, and has its maximal effect about one hour after it is taken.

  • When used to treat insomnia, it is prescribed at bedtime. Indivduals over the age of 60 years are especially sensitive to the sedating and anticholinergic effects of diphenhydramine, and the dose should be reduced.
  • A common regimen for treating adult allergic reaction is 25-50 mg every 4-6 hours not to exceed 300 mg daily.
  • Insomnia is treated with 50 mg taken 30 minutes before bedtime.

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 (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, Endep], imipramine [Tofranil], desipramine [Norpramin]), and certain antihypertensive medications (for example, clonidine [Catapres], propranolol [Inderal]). Diphenhydramine also can intensify the drying effects of other medications with anticholinergic properties (for example, dicyclomine [Bentyl] and bethanechol [Urecholine], probanthine).

Medically Reviewed by a Doctor on 7/28/2014



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