First-generation sedating antihistamine
ATC code: R06AA02 (Diphenhydramine)
Blocks H1 histamine receptors. Crosses BBB, causes marked sedation. Anticholinergic activity explains dry mouth and urinary retention in elderly. Used in acute allergic reactions, short-term insomnia, motion sickness.
Second line
Urticaria and acute allergic reaction. Per 2022 – second-line after non-sedating 2nd-generation H1-blockers. Used for severe acute reactions or angioedema, 25-50 mg orally or IM up to 4 times daily. Long-term use not recommended due to sedation and anticholinergic effects.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
Not recommended
Diphenhydramine while breastfeeding. RID about 1%, Hale L2. A single low dose is safe. Repeated high doses may theoretically reduce milk supply (anticholinergic) and cause infant drowsiness. Prefer loratadine (Hale L1) or cetirizine.
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Safe when indicated. Long-term high-dose use in 3rd trimester – withdrawal and paradoxical excitation in newborn.
Compatible with caution. Hale L2. RID about 1%. Single dose – safe. Repeated use risks milk supply reduction and infant drowsiness. Prefer loratadine, cetirizine.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.
Diphenhydramine is evaluated for the following indications with varying evidence strength: Urticaria (evidence tier B), Allergy during breastfeeding (evidence tier C). See the full indication matrix with dosing and citations above on this page.
Common side effects of Diphenhydramine (≥ 1 in 100): Drowsiness, Dry mouth, Constipation, Urinary retention, Dizziness. See the Safety section for uncommon and serious reactions.
FDA category B. Safe when indicated. Long-term high-dose use in 3rd trimester – withdrawal and paradoxical excitation in newborn.
Compatible with caution. Hale L2. RID about 1%. Single dose – safe. Repeated use risks milk supply reduction and infant drowsiness. Prefer loratadine, cetirizine.
Diphenhydramine is contraindicated in: Hypersensitivity; Narrow-angle glaucoma; Prostatic hypertrophy with urinary retention; Stenosing GI ulcer; Age <2 years. Full list in the Safety section.