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Diphenhydramine: Difference between revisions

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{{MedTemplate
{{MedTemplate
| generic           = Diphenhydramine
| generic           = Diphenhydramine (hydrochloride; citrate)
| brand             = Benadryl
| brand             = Benadryl (oral, injectable, topical), Banophen, Sominex (sleep aid), ZzzQuil, Aler-Dryl
| structure         =  
| structure         =
| classes           = Antihistamine, Deliriant
| classes           = [[:Category:Antihistamines|First-generation antihistamine (ethanolamine)]], [[:Category:Anticholinergics|Anticholinergic]], [[:Category:Sleep aids|Sleep aid]], [[:Category:Antiemetics|Antiemetic]]
| mechanism          = H1 antagonist; muscarinic antagonist
| uses              = <vote slug="allergic-conditions-use">Allergic conditions including urticaria and seasonal allergies (OTC and FDA)</vote>, <vote slug="motion-sickness-use">Motion sickness (OTC, FDA)</vote>, <vote slug="insomnia-otc-use">Insomnia, short-term (OTC, FDA)</vote>, <vote slug="acute-dystonia-emergency-use">Acute dystonic reaction from neuroleptics (off-label, IV emergency)</vote>, <vote slug="anaphylaxis-adjunct-use">Anaphylaxis adjunct (FDA, IV; not a substitute for epinephrine)</vote>, <vote slug="parkinson-disease-anticholinergic-use">Parkinson disease tremor (off-label, anticholinergic effect)</vote>
| uses              =  
| 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
| starting_dose     =  
| preparations     = Tablets 25, 50 mg; capsules; liquid; chewable; topical cream/gel; injection 50 mg/mL
| preparations      =  
| fda_max          = 300 mg/day (oral)
| fda_max           =  
| pill_id           =
| routes             =  
| routes           = Oral, IV, IM, topical
| onset             =  
| onset             = 15-30 minutes (oral); 1-2 minutes (IV)
| duration           =  
| duration         = 4-6 hours
| halflife           =  
| halflife         = 4-8 hours (longer in elderly, 9-13 hours)<ref name="benadryl-label">FDA Prescribing Information, Benadryl (diphenhydramine hydrochloride), Johnson & Johnson/various, current revision. https://www.accessdata.fda.gov/drugsatfda_docs/label/2007/010661s045lbl.pdf</ref>
| bioavailability   =  
| bioavailability   = ~40-60% (oral, with significant first-pass)<ref name="benadryl-label" />
| pregnancy         =  
| pregnancy         = Older agent with substantial use experience; broadly considered safe in pregnancy<ref name="benadryl-label" />
| legal             =  
| legal             = [[USLegal:Over-the-counter|OTC]] for most oral and topical formulations; [[USLegal:Prescription only|Rx-only]] for injectable
| intro              =
| mechanism        = <vote slug="diphenhydramine-mech-claim">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").</vote> 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<ref name="benadryl-label" />.
| pharmacokinetics  =
<effect ref="dry-mouth" author="MDElliottMD"/>
| pharmacodynamics  =  
| indications        =  
| dosing            =
| effects           =
| interactions      = <pharmaInteractions/>
| pregnancy_details  =  
| monitoring        =  
| counseling        =
| anecdotes          =
| seealso            =
| references        =  
}}
}}
== References ==
<references />


[[Category:Antihistamines]]
[[Category:Antihistamines]]
[[Category:Anticholinergics]]
[[Category:Sleep aids]]
[[Category:Antiemetics]]

Latest revision as of 07:32, 23 May 2026

First-generation antihistamine (ethanolamine), Anticholinergic, Sleep aid, Antiemetic
Diphenhydramine (hydrochloride; citrate)
Benadryl (oral, injectable, topical), Banophen, Sominex (sleep aid), ZzzQuil, Aler-Dryl

Experience

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Problems

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Titration strategies

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Effects

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Relevant Literature

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Summary
Classes
First-generation antihistamine (ethanolamine), Anticholinergic, Sleep aid, Antiemetic
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]
Legal status
OTC for most oral and topical formulations; Rx-only for injectable
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

References

  1. ↑ 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
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