Generic Name
Diphenhydramine
Group
First-generation antihistamine (H1 receptor antagonist)
Mechanism of Action
Blocks central and peripheral H1 histamine receptors, preventing histamine-mediated allergic responses and causing sedation due to CNS penetration.
Indications
- Allergic rhinitis
- Urticaria (hives)
- Acute anaphylaxis (as part of emergency treatment)
- Motion sickness, nausea, and vomiting
- Insomnia (short-term use)
Dose
- Adults: 25–50 mg orally every 4–6 hours (max 300 mg/day)
- Children 6–12 years: 12.5–25 mg orally every 4–6 hours
- Children 2–6 years: 6.25 mg orally every 4–6 hours
- IV/IM (emergency): 25–50 mg, may repeat every 6–8 hours
Major Side Effects
- Drowsiness, sedation
- Dizziness, confusion (especially in elderly)
- Dry mouth, blurred vision
- Gastrointestinal upset (nausea, vomiting)
- Paradoxical excitation in children
Contraindications
- Hypersensitivity to diphenhydramine
- Newborns, premature infants (risk of CNS depression)
- Use with caution in patients with glaucoma, prostatic hypertrophy, or asthma
Compatible Fluid
- Oral: water, juice
- IV: compatible with 0.9% normal saline or 5% dextrose
Route of Administration
- Oral (tablet, syrup, capsule)
- IV/IM for emergency use
Type of Administration
Used acutely for allergic reactions, motion sickness, or as a sedative; can be used chronically for allergies under supervision.