Diphenhydramine
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Diphenhydramine (hydrochloride; citrate)
Benadryl (oral, injectable, topical), Banophen, Sominex (sleep aid), ZzzQuil, Aler-Dryl
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Summary
Common uses
Allergic conditions including urticaria and seasonal allergies (OTC and FDA)0, Motion sickness (OTC, FDA)0, Insomnia, short-term (OTC, FDA)0, Acute dystonic reaction from neuroleptics (off-label, IV emergency)0, Anaphylaxis adjunct (FDA, IV; not a substitute for epinephrine)0, Parkinson disease tremor (off-label, anticholinergic effect)0
Pharmacy
Starting dose
Allergy: 25-50 mg PO every 4-6 hours. Insomnia: 25-50 mg PO at bedtime. IV (acute dystonia, severe allergic reaction): 25-50 mg slow IV push
Preparations
Tablets 25, 50 mg; capsules; liquid; chewable; topical cream/gel; injection 50 mg/mL
US FDA Max
300 mg/day (oral)
Pharmacology
Routes
Oral, IV, IM, topical
Onset
15-30 minutes (oral); 1-2 minutes (IV)
Duration
4-6 hours
Half-life
4-8 hours (longer in elderly, 9-13 hours)[1]
Bioavailability
~40-60% (oral, with significant first-pass)[1]
Pregnancy
Older agent with substantial use experience; broadly considered safe in pregnancy[1]
Purported mechanism
First-generation H1 histamine receptor antagonist with substantial central nervous system penetration and strong muscarinic acetylcholine receptor antagonism. The H1 blockade produces antihistamine and antiemetic effects; the muscarinic blockade produces sedation, anticholinergic burden (dry mouth, blurred vision, urinary retention, cognitive impairment), and at supratherapeutic doses, anticholinergic delirium with classic toxidrome ("blind as a bat, mad as a hatter").0 Major Beers-list concern in elderly patients for cognitive and fall risks. CYP2D6 substrate. At massive overdose, also produces sodium channel blockade with cardiac toxicity[1].
Dry mouth 100% -67.0 (n=1) no reports yet
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References
- ↑ 1.0 1.1 1.2 1.3 FDA Prescribing Information, Benadryl (diphenhydramine hydrochloride), Johnson & Johnson/various, current revision. https://www.accessdata.fda.gov/drugsatfda_docs/label/2007/010661s045lbl.pdf