Diphenhydramine injection
BENADRYL IM / IV
Pronunciation: dye-fen-HYE-dra-meen
“Dippin’ hydra’s mean Bean drill”
“wears two hats” – max antihistamine and antimuscarinic
What it isInjectable H1 and muscarinic antagonist; allergic reactions and acute dystonia or parkinsonism.
Why this oneB-52 ingredient: anticholinergic cover against haloperidol dystonia while it sedates.
What limits itSedation, delirium, urinary retention, and hypotension make its anticholinergic burden especially hazardous in older adults.
FDA-approved for
- anaphylaxis adjunct / allergic reactions (~1946)
- dystonia / parkinsonism (~1946)
Principal riskAnticholinergic burden; on every elderly deprescribing list
Classes and tags
Clinical profile
Legacy pregnancy categoryB
Defining liabilitySedation and impaired alertness, anticholinergic effects, and additive effects with alcohol or other CNS depressants; overdose can be serious. Injection: hypotension, especially in older adults.
FDA label
Read the full label on DailyMed ↗
NO BOXED WARNINGS
| FDA dosing | Population | Start | Target | Max |
|---|---|---|---|---|
| Allergic reactions/anaphylaxis adjunct, motion sickness, parkinsonism (when oral impractical) | Adults | 10–50 mg IV/IM (up to 100 mg if required) | 400 mg/day | |
| Allergic reactions, motion sickness, parkinsonism (when oral impractical) | Pediatric (not neonates/premature infants) | 5 mg/kg/day or 150 mg/m²/day IV/IM divided into 4 doses | 300 mg/day |
Renal Undefined.
Hepatic Undefined.
Instructions
- IV or IM only; IV rate generally not exceeding 25 mg/min
- Administer deep intramuscularly
- Do not use subcutaneously/intradermally (risk of necrosis)
Available strengths
- inj 50 mg/mL
- cap 25, 50 mg
- tab 25, 50 mg
- chew tab 12.5, 25 mg
- sol 12.5 mg/5 mL
- gel 0.4 g/20 g
- gel 1.14 g/57 g
- gel 12.5, 25 mg
- + 5 other marketed forms not shown
Contraindications
- Use in neonates or premature infants
- Use in nursing mothers
- Use as a local anesthetic (risk of local necrosis)
Cautions
- Caution in narrow-angle glaucoma, stenosing peptic ulcer, pyloroduodenal/bladder-neck obstruction, prostatic hypertrophy
- Overdose in pediatrics may cause hallucinations, convulsions, death
- More sedation/dizziness/hypotension in the elderly
Adverse reactions
- Sedation/sleepiness
- Dizziness
- Hypotension
- Dry mouth/nose/throat
- Tachycardia
- Thickening of bronchial secretions
- Urinary retention
Pregnancy Undefined.
Lactation Contraindicated in nursing mothers (higher antihistamine risk for infants).
Cost
| Strength | #/mo | generic $/unit | CostPlus | |
|---|---|---|---|---|
| 50 mg/mL | inj | per unit | $0.90/mL | not offered |
Other forms: Diphenhydramine (BENADRYL)
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Memory aids: interaction avatars
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Self-check
- What role does diphenhydramine serve in the B-52 combination?
Anticholinergic cover against haloperidol dystonia while sedating. - Who is especially vulnerable to its anticholinergic burden?
Older adults.