NSTK · 01.2026Independent supplement reference
NutriStack
Edition 1.0Updated July 13, 2026

Diphenhydramine

Prescription ·Exploratory record · not publication-reviewed ·Updated July 13, 2026

A first-generation antihistamine and the prototypical H1 receptor antagonist. Used for allergic reactions (urticaria, angioedema, allergic rhinitis), motion sickness, insomnia (OTC sleep aid), acute dystonic reactions, and as an adjunct in anaphylaxis. Readily crosses the blood-brain barrier, causing significant sedation and anticholinergic effects. Widely available over-the-counter. Due to sedation, cognitive impairment, and anticholinergic burden, second-generation antihistamines are generally preferred for chronic allergic conditions.

Evidence tier
Strong
Indexed sources
11 2003–2026
Interactions
0
Category
Prescription
Read the sources → Compare profiles
Publication status

Exploratory record, not a reviewed medical publication.

See the editorial and medical-review policies for the publication gate.

This generated database record has not completed a claim-level review by a qualified human medical reviewer. Bibliography links and evidence labels are not approval. The page remains available for reference and is excluded from search indexing.

In short

What the research says.

A quick, data-only summary. The full evidence, dosing, and sources are below.

A first-generation antihistamine and the prototypical H1 receptor antagonist. NutriStack rates the supporting evidence as strong. Common adult dosing is 25-50 mg every 4-6 hours (max 300 mg/day); sleep: 25-50 mg at bedtime; IV/IM for severe allergic reactions: 10-50 mg (as prescribed by your physician), though a prescriber sets the actual dose. This profile indexes 11 sources.

What it's good for
  • Rapid relief of acute allergic reactions (urticaria, angioedema)2,3
  • Adjunctive treatment in anaphylaxis (with epinephrine)10
  • OTC sleep aid
  • Treats motion sickness and nausea
  • Treats acute dystonic reactions (drug-induced)2,3
What to watch for
  • Drowsiness and sedation (pronounced)
  • Dry mouth, nose, and throat
  • Urinary retention
  • Known hypersensitivity to diphenhydramine1,2
  • Neonates and premature infants

The bottom line

Evidence rating strong. Most-documented uses: rapid relief of acute allergic reactions (urticaria, angioedema), adjunctive treatment in anaphylaxis (with epinephrine), otc sleep aid. 11 sources indexed (2003–2026), with 0 interaction records on file.

The science

How it works, mechanistically.

Core mechanism

Competitively blocks H1 histamine receptors on effector cells (smooth muscle, vascular endothelium, respiratory epithelium). Unlike second-generation antihistamines, it readily crosses the blood-brain barrier and blocks central H1 receptors, causing sedation and suppression of the emetic/vestibular center (antiemetic/anti-motion-sickness effects). Also has significant muscarinic (anticholinergic) receptor blocking activity, contributing to dry mouth, urinary retention, constipation, and delirium risk in elderly. Additional sodium channel blocking activity provides local anesthetic effects.

Class
First-Generation Antihistamine
Dosing

Dosing & protocol.

Common range
25-50 mg every 4-6 hours (max 300 mg/day); sleep: 25-50 mg at bedtime; IV/IM for severe allergic reactions: 10-50 mg (as prescribed by your physician)
Recommended form
Oral tablets, capsules, liquid, IV, or IM injection; topical cream (limited systemic use)

Well absorbed orally with bioavailability 40-60% (first-pass metabolism). Onset of oral sedation within 15-30 minutes. Can be taken with food to reduce GI upset. Avoid use in elderly due to Beers Criteria anticholinergic risk.

Safety

Full safety detail.

Side effects

  • Drowsiness and sedation (pronounced)
  • Dry mouth, nose, and throat
  • Urinary retention
  • Constipation
  • Blurred vision (mydriasis, cycloplegia)
  • Dizziness and impaired coordination
  • Cognitive impairment (especially in elderly)
  • Paradoxical excitation (especially in children)

Contraindications

  • Known hypersensitivity to diphenhydramine1,2
  • Neonates and premature infants
  • Nursing mothers (excreted in breast milk; may inhibit lactation)
  • Narrow-angle glaucoma
  • Urinary retention/bladder neck obstruction
  • Elderly patients (Beers Criteria, increased risk of confusion, falls, urinary retention)9
Sources

Sources, by evidence tier.

Numbered references. Citations throughout the page link here.

Meta-analyses & systematic reviews

1

Randomized controlled trials

3

Reviews & position papers

3

Reference material

4
FAQ

Common questions.

Quick answers about Diphenhydramine, drawn from the data on this page.

What is Diphenhydramine?

A first-generation antihistamine and the prototypical H1 receptor antagonist. Used for allergic reactions (urticaria, angioedema, allergic rhinitis), motion sickness, insomnia (OTC sleep aid), acute dystonic reactions, and as an adjunct in anaphylaxis. Readily crosses the blood-brain barrier, causing significant sedation and anticholinergic effects. Widely available over-the-counter. Due to sedation, cognitive impairment, and anticholinergic burden, second-generation antihistamines are generally preferred for chronic allergic conditions.

What is Diphenhydramine used for?

Commonly cited benefits of Diphenhydramine include: Rapid relief of acute allergic reactions (urticaria, angioedema); Adjunctive treatment in anaphylaxis (with epinephrine); OTC sleep aid; Treats motion sickness and nausea; Treats acute dystonic reactions (drug-induced). NutriStack rates the overall evidence as strong.

What is a typical dose of Diphenhydramine?

A typical adult dose of Diphenhydramine is 25-50 mg every 4-6 hours (max 300 mg/day); sleep: 25-50 mg at bedtime; IV/IM for severe allergic reactions: 10-50 mg (as prescribed by your physician). The commonly recommended form is Oral tablets, capsules, liquid, IV, or IM injection; topical cream (limited systemic use). These are general ranges, not personalized advice; confirm the right dose with a clinician.

What are the side effects of Diphenhydramine?

Reported side effects include drowsiness and sedation (pronounced), dry mouth, nose, and throat, urinary retention, constipation, blurred vision (mydriasis, cycloplegia), dizziness and impaired coordination. This is educational information; check with a healthcare professional before use.

How strong is the evidence for Diphenhydramine?

NutriStack rates this as Strong, meaning multiple high-quality meta-analyses or systematic reviews converge on the same finding. Full tier criteria are at https://nutristackapp.com/methodology/evidence-tiers.

Use this with your stack

Diphenhydramine in NutriStack.

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