diphenhydramine hydrochloride

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diphenhydramine hydrochloride

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Pharmacologic class: Ethanolamine derivative, nonselective histamine1-receptor antagonist

Therapeutic class: Antihistamine, antitussive, antiemetic, antivertigo agent, antidyskinetic

Pregnancy risk category B


Interferes with histamine effects at histamine1-receptor sites; prevents but doesn't reverse histamine-mediated response. Also possesses CNS depressant and anticholinergic properties.


Capsules: 25 mg, 50 mg

Elixir: 12.5 mg/5 ml

Injection: 10 mg/ml, 50 mg/ml

Strips (orally disintegrating): 12.5 mg, 25 mg

Syrup: 12.5 mg/5 ml

Tablets: 25 mg, 50 mg

Tablets (chewable): 12.5 mg, 25 mg

Tablets (orally disintegrating): 12.5 mg

Indications and dosages

Allergy symptoms caused by histamine release (including anaphylaxis, seasonal and perennial allergic rhinitis, and allergic dermatoses); nausea; vertigo

Adults and children over age 12: 25 to 50 mg P.O. q 4 to 6 hours, or 10 to 50 mg I.V. or I.M. q 2 to 3 hours p.r.n. (Some patients may need up to 100 mg.) Don't exceed 400 mg/day.

Children ages 6 to 12: 12.5 to 25 mg P.O. q 4 to 6 hours, or 1.25 mg/kg (37.5 mg/m2) I.M. or I.V. q.i.d. Don't exceed 150 mg/day.

Children ages 2 to 5: 6.25 mg P.O. q 4 to 6 hours. Don't exceed 37.5 mg/day.


Adults: 25 mg P.O. q 4 hours p.r.n. Don't exceed 150 mg/day.

Children ages 6 to 12: 12.5 mg P.O. q 4 hours. Don't exceed 75 mg/day.

Children ages 2 to 5: 6.25 mg P.O. q 4 hours. Don't exceed 37.5 mg/24 hours.

Dyskinesia; Parkinson's disease

Adults: Initially, 25 mg P.O. t.i.d.; may be increased to a maximum of 50 mg q.i.d.

Mild nighttime sedation

Adults: 50 mg P.O. 20 to 30 minutes before bedtime

Dosage adjustment

• Elderly patients

Off-label uses

• Drug-induced extrapyramidal reactions


• Hypersensitivity to drug
• Alcohol intolerance
• Acute asthma attacks
• MAO inhibitor use within past 14 days
• Breastfeeding
• Neonates, premature infants


Use cautiously in:
• severe hepatic disease, angle-closure glaucoma, seizure disorders, prostatic hypertrophy, cardiovascular disease, hyperthyroidism
• elderly patients
• pregnant patients (safety not established)
• children younger than age 2 (safety not established).


• For motion sickness, administer 30 minutes before activity.
• For I.V. use, check compatibility before mixing with other drugs.
• Inject I.M. dose deep into large muscle mass; rotate sites.
• Discontinue drug 4 days before allergy skin testing to avoid misleading results.

Don't give within 14 days of MAO inhibitors.

Adverse reactions

CNS: drowsiness, dizziness, headache, paradoxical stimulation (especially in children)

CV: hypotension, palpitations, tachycardia

EENT: blurred vision, tinnitus

GI: diarrhea, constipation, dry mouth

GU: dysuria, urinary frequency or retention

Skin: photosensitivity

Other: decreased appetite, pain at I.M. injection site


Drug-drug.Antihistamines, opioids, sedative-hypnotics: additive CNS depression

Disopyramide, quinidine, tricyclic antidepressants: increased anticholinergic effects

MAO inhibitors: intensified and prolonged anticholinergic effects

Drug-diagnostic tests.Skin allergy tests: false-negative results

Hemoglobin, platelets: decreased values

Drug-herbs.Angel's trumpet, jimson weed, scopolia: increased anticholinergic effects

Chamomile, hops, kava, skullcap, valerian: increased CNS depression

Drug-behaviors.Alcohol use: increased CNS depression

Patient monitoring

• Monitor cardiovascular status, especially in patients with cardiovascular disease.
• Supervise patient during ambulation. Use side rails as necessary.

Patient teaching

• Advise patient to avoid alcohol and other depressants such as sedatives while taking drug.
• Caution patient to avoid driving and other hazardous activities until he knows how drug affects concentration and alertness.
• As appropriate, review all other significant adverse reactions and interactions, especially those related to the drugs, tests, herbs, and behaviors mentioned above.

di·phen·hy·dra·mine hy·dro·chlor·ide

(dī'fen-hī'dră-mēn hī'drō-klōr'īd),
An H2 antihistaminic with anticholinergic and sedative properties.

diphenhydrAMINE hydrochloride

an antihistamine.
indications It is prescribed in the treatment of hypersensitivity reactions, including rhinitis, skin rash, and pruritus, and in the treatment of motion sickness and insomnia.
contraindications Asthma or known hypersensitivity to this drug prohibits its use. It is not given to newborns or lactating mothers.
adverse effects Among the more serious adverse reactions are skin rash, hypersensitivity reactions, and tachycardia. Drowsiness and dry mouth commonly occur.