RESEARCH MONOGRAPH · KDC-MN-354
Diphenhydramine
Diphenhydramine, sold as Benadryl, is the canonical first-generation antihistamine and the active ingredient in most over-the-counter sleep aids (ZzzQuil, Tylenol PM, Unisom Sleepminis). It blocks H1 receptors for allergy relief but also blocks muscarinic receptors, producing the classic anticholinergic side effect cluster (dry mouth, urinary retention, constipation, confusion). Long-term use, particularly in older adults, has been linked to cognitive impairment and possibly increased dementia risk in observational studies. Newer non-sedating antihistamines like cetirizine or loratadine are preferred for routine allergy control; diphenhydramine remains useful for acute allergic reactions and as an antiemetic. Not stocked by Kodiac. This monograph is provided for research and educational reference.
Intrigue 0–100 blends mechanism novelty, evidence strength, and translational potential. Kodiac editorial, not peer-reviewed.
First-generation H1 antihistamine + anticholinergic
An ethanolamine first-generation H1 antihistamine; the canonical sedating antihistamine and OTC sleep aid.
Abstract
Diphenhydramine (2-(diphenylmethoxy)-N,N-dimethylethanamine; CAS 58-73-1; molecular formula C17H21NO; molecular weight 255.36) is an ethanolamine first-generation H1 antihistamine developed at Parke-Davis and approved in 1946. The compound is the canonical sedating H1 blocker (Ki approximately 16 nM) with significant anticholinergic activity (M1-M5 antagonism, Ki approximately 70 nM). The combined activity produces sedation, dry mouth, blurred vision, and confusion (especially in elderly). Plasma half-life is approximately 4 to 8 hours. Approved indications: allergic conditions, motion sickness, parkinsonism, anaphylaxis. OTC sleep aid use in branded products (Tylenol PM, ZzzQuil) is widespread despite limited efficacy and concerning anticholinergic burden in elderly (associated with dementia risk in long-term use per BEERS criteria). The compound has dose-dependent abuse potential at high doses (deliriant effect from anticholinergic activity). Used as the canonical first-generation H1 antihistamine in research.
Mechanism of action
First-generation H1 antagonism with significant muscarinic antagonism. Sedation, dry mouth, anticholinergic effects.
Reported research dose ranges
Clinical 25 to 50 mg in the published literature every 4 to 6 hours (max 300 mg).
References
- Simons FE. Advances in H1-antihistamines. N Engl J Med 2004.
- Glass JR, et al. Acute pharmacological effects of temazepam, diphenhydramine, and valerian in healthy elderly subjects. J Clin Psychopharmacol 2003.
- Gray SL, et al. Cumulative use of strong anticholinergics and incident dementia. JAMA Intern Med 2015.
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The full reference document is provided strictly for research use only. It reports research dose ranges from the published literature, not instructions for use in humans or animals.
FOR RESEARCH USE ONLY. Not for medical, diagnostic, or therapeutic purposes. Not for human consumption. All information is provided for research and educational purposes only.