Name

diphenhydramine (Benadryl®)

Class

Antihistamine (H1 only)

Description/Mechanism

An antihistamine with significant anticholinergic effects and a high incidence of drowsiness. Competes for H1 receptors on effector cells thus blocking effects of histamine release. Effects in parkinsonism and drug-induced extrapyramidal symptoms are apparently related to its ability to suppress central cholinergic activity and to prologn action of dopamine by inhibiting its reuptake and storage. Diphenhydramine does not effect H2 receptors and therefore has no effect on gastric acid secretion.

Onset

Maximal effects in 1-3 hours

Duration

6-12 hours

Indications

allergic reactions (after epinephrine)

Anaphylaxis (after epinephrine)

Acute extrapyramidal reactions

Contraindications

Acute asthma

CNS depression

Patients taking monoamine oxidase (MAO) inhibitors

Hpersensitivity

Adverse Reactions

Dose-related drowsiness

Sedation

Disturbed coordination

Hypotension

Palpitations

Tachycardia, bradycardia

Thickening of bronchial secretions

Dry mouth and throat

Drug Interactions

CNS depressants may increase depressant effects

MAO inhibitors may prolong and intensify anticholinergic effects of antihistamines.

Supplied

10,50 mg/ml vials, prefilled syringes

Dose/Administration

Adult

25-50 mg IM, IV

Pediatric

1-2 mg/kg IV, IM (max 50 mg)

Special Consideration

Pregnancy Safety: Category B