GENERIC NAME: diphenhydramine

BRAND NAME: Benadryl

DRUG CLASS AND MECHANISM: Diphenhydramine is an antihistamine with anticholinergic (drying) and sedative properties that is used to treat 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 causes 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 was originally approved by the FDA in 1946.

GENERIC AVAILABLE: yes

PRESCRIPTION: no

PREPARATIONS: Capsules: 25mg, 50mg; Elixer: 12.5mg per teaspoon (5mL)

STORAGE: Capsules should be stored below 30°C (86°F).

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 treat patients with chronic urticaria. 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 also is used as an aid for insomnia.

DOSING: Diphenhydramine has its maximal effect about one hour after it is taken. Its effects last for 4 to 6 hours. Therefore, it is often prescribed to be taken every 4 to 6 hours as needed for relief of allergy-related symptoms. When used to combat insomnia, it is prescribed to be taken 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.

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 (e.g., Valium, Ativan, Klonopin, Xanax), the narcotic class of pain medications and its derivatives (e.g., Percocet, Vicodin, Dilaudid, Codeine , Darvon), the tricyclic class of antidepressants (e.g., Elavil, Tofranil, Norpramin), and certain antihypertensive medications (e.g., Catapres, Inderal). Diphenhydramine can also intensify the drying effects of other medications with anticholinergic properties (e.g., Bentyl, Urecholine, Probanthine).

PREGNANCY: Diphenhydramine has been used extensively in pregnant women; the incidence of fetal malformations is the same as that which would be expected in women who are not taking medications. Additionally, studies in animals suggest no important effects on the fetus. Diphenhydramine, therefore, can be used in pregnancy if the physician feels that it is needed.

NURSING MOTHERS: Diphenhydramine is secreted in breast milk. Because of the risk of stimulation and seizures in infants -- especially newborns and prematures -- 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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