Diphenhydramine

Question 1

Class

Answer 1

Antihistamine

Question 2

Description

Answer 2

Prevents physiological actions of histamine by blocking histamine-1 (H1; eg, diphenhydramine, cimetidine) & H2 (eg, cimetidine, ranitidine, famotidine) receptor sites

Used for histamine excessive conditions (eg, acute urticaria)

Used as adjunctive therapy (eg, w/ Epi) in the tx of anaphylactic shock

Tx for certain extrapyramidal (dystonic) reactions

Upper respiratory tract relief & sinus symptoms associated w/ allergic reactions

Question 3

Onset & Duration

Answer 3

Onset: max effects 1-3 hours

Duration: 6-12 hours

Question 4

Indications

Answer 4

Mod-severe allergic reactions

Anaphylaxis (after epi)

Acute extrapyramidal (dystonic) reactions

Question 5

Contraindications

Answer 5

Pts taking nonselective MAO inhibitors (now increasingly rare)

Hypersensitivity

Narrow-angle glaucoma (relative)

Newborns & nursing mothers (per manufacturer)

Question 6

Adverse Reactions

Answer 6

Hypotension

Dose-related drowsiness

Disturbed coordination

Dry mouth & throat

Tachycardia, bradycardia

Thickening of bronchial secretions

Paradoxical excitement in children

Palpitations

Question 7

Drug Interactions

Answer 7

CNS depressants may increase depressant effects

MAO inhibitors may prolong & intensity anticholinergic effects of antihistamines

Question 8

How supplied

Answer 8

Parenteral:
10- & 50-mg/mL vials, prefilled syringe

Question 9

Dosage & Administration

Answer 9

Adult:
1 mg/kg IM/PO or slow IV (50mg max dose)

Ped:
1 mg/kg IM/IV/PO (25mg max dose)

Question 10

Special Considerations

Answer 10

Pregnancy safety: Category C

Practice caution in pts w/ CNS depression or lower respiratory tract disease such as asthma