Classifications: antihistamine; h1-receptor antagonist;
Therapeutic: antihistamine
; h1-receptor antagonist
Prototype: Diphenhydramine
Pregnancy Category: C


10 mg/mL injection; 2 mg/5 mL elixir; 4 mg tablet; ingredient in many oral combination products containing a decongestant, expectorant, and/or analgesic


Antihistamine that has less sedative effect than diphenhydramine. Competes with histamine for H1-receptor sites on effector cells in the bronchi and bronchioles, thus blocking histamine-mediated responses.

Therapeutic Effect

Effective against upper respiratory symptoms and allergic manifestations.


Symptomatic treatment of allergic manifestations. Also used in various cough mixtures and antihistamine-decongestant cold formulations.


Hypersensitivity to antihistamines; acute asthma; pregnancy (category C), newborns.

Cautious Use

Older adults; prostatic hypertrophy; GI obstruction; asthma; narrow-angle glaucoma; COPD, cardiovascular or renal disease; seizure disorders; hyperthyroidism; lactation.

Route & Dosage

Adult: PO 4–8 mg t.i.d. or q.i.d. or 8–12 mg of sustained release  b.i.d. or t.i.d. SC/IM/IV 5–20 mg q6–12h (max: 40 mg/24 h)
Geriatric: PO 4 mg 1–2 times/d
Child: PO >6 y, 2–4 mg t.i.d. or q.i.d. or 8–12 mg of sustained release b.i.d. (max: 12 mg/24 h); <6 y, 0.5 mg/kg in 3–4 divided doses


  • Give with meals or a snack to prevent gastric irritation.
  • Give without further dilution or diluted to a 1:10 ratio with NS.

PREPARE: Direct: Give undiluted or diluted with 10 mL D5W or NS.  

ADMINISTER: Direct: Give IV push slowly over 1 min to a recumbent patient.  

INCOMPATIBILITIES Solution/additive: Radio-contrast media (diatrizoate, iothalamate), insulin, pentobarbital.

  • Store in tightly covered container at 15°–30° C (59°–86° F) unless otherwise directed. Elixir and parenteral form should be protected from light. Avoid freezing.

Adverse Effects (≥1%)

Body as a Whole: Hypersensitivity reaction (urticaria, increased sweating, agranulocytosis). CNS: Sedation, drowsiness, dizziness, headache, disturbed coordination. GI: Dry mouth, throat, and nose, stomach upset, constipation. Special Senses: Ringing or buzzing in ears. Skin: Rash, photosensitivity.

Diagnostic Test Interference

May cause false-negative allergy skin tests.


Drug: Alcohol and other cns depressants add to sedation.


Peak: 3–9 h. Duration: Up to 48 h. Distribution: Crosses placenta. Elimination: 40% in urine within 72 h; 2% in feces. Half-Life: 12–34 h.

Nursing Implications

Assessment & Drug Effects

  • Drowsiness, sweating, transient hypotension, and syncope may follow IV administration; reaction to drug should be evaluated. Keep physician informed.
  • Note: Older adults tend to be particularly susceptible to drug's sedative effect, dizziness, and hypotension. Most symptoms respond to reduction in dosage.
  • Lab tests: Periodic CBC in patients receiving long-term therapy.

Patient & Family Education

  • Acute hypersensitivity reaction can occur within minutes to hours after drug ingestion. Reaction is manifested by high fever, chills, and possible development of ulcerations of mouth and throat, pneumonia, and prostration. Patient should seek medical attention immediately.
  • Follow diligent mouth care. Sugarless gum, lemon drops, or frequent rinses with warm water may relieve dry mouth.
  • Do not drive a car or perform other potentially hazardous activities until response to drug is known.
  • Do not take alcoholic beverages or other CNS depressants (e.g., tranquilizers, sedatives, pain or sleeping medicines) without consulting physician.

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug

© 2006-2022 Last Updated On: 11/26/2022 (0)
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