Medications and Drugs
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].
Report Problems to the Food and Drug Administration
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.
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