Diphenhydramine

What is Diphenhydramine?

What diphenhydramine is

Diphenhydramine is a first-generation H1 antihistamine introduced in 1946 — one of the oldest OTC medications still in widespread use. It blocks histamine action at H1 receptors (reducing allergy symptoms) and crosses the blood-brain barrier (causing drowsiness). Also has anticholinergic effects useful for nausea and motion sickness1.

Common brand uses include:

• Benadryl — allergies, hives, itching
• Sominex, Unisom Sleeptabs (diphenhydramine version) — sleep aid
• Common ingredient in nighttime cold/flu products (Tylenol PM, etc.)

Why diphenhydramine is pregnancy-safe

• 80+ years of clinical use. Used extensively in pregnancy since the 1940s without consistent evidence of teratogenicity2.
• FDA Pregnancy Category B. Animal studies have not demonstrated risk; human pregnancy data are reassuring.
• Mainstream pregnancy endorsement. ACOG, the Society of Obstetricians and Gynaecologists of Canada, and the American Academy of Allergy, Asthma, and Immunology all consider diphenhydramine acceptable in pregnancy3.
• The doxylamine equivalent in Diclegis. The FDA-approved pregnancy nausea medication Diclegis (doxylamine + vitamin B6) is essentially a related first-generation antihistamine combined with B6. The pattern (first-gen antihistamine + B6) was the historical “Bendectin” formulation widely used for morning sickness for decades.

Common pregnancy uses

• Allergy relief. Better for nighttime use (drowsiness is sometimes a feature, not a bug). For daytime allergies, second-generation antihistamines (loratadine, cetirizine) are preferred.
• Sleep aid. The drowsiness side effect makes it useful as an occasional pregnancy sleep aid. Not for chronic insomnia.
• Pregnancy nausea adjunct. Doxylamine + B6 (Diclegis or Unisom-with-B6) is the more common nausea combination, but diphenhydramine has been used similarly. Discuss with your OB.
• Itching from pregnancy-related conditions. Pruritic urticarial papules and plaques of pregnancy (PUPPP) and similar conditions; topical antihistamine creams may complement oral diphenhydramine.

Caveats and practical guidance

• Drowsiness: Don’t drive or operate machinery if recently dosed. Be aware that even smaller doses can affect daytime alertness.
• Dry mouth, constipation. Anticholinergic side effects can worsen pregnancy constipation; balance benefit vs. trade-off.
• Late pregnancy. Some references recommend caution in the final 2 weeks before delivery due to theoretical neonatal effects, though clinical concern is minimal.
• Combination products. Many OTC products contain diphenhydramine combined with NSAIDs (Advil PM contains ibuprofen + diphenhydramine) — avoid combination products after 20 weeks due to the NSAID component4.

Breastfeeding considerations

Diphenhydramine reaches breast milk and can cause infant drowsiness. It can also reduce maternal milk supply through anticholinergic effects. Intermittent use is acceptable; for chronic allergy management while nursing, second-generation antihistamines (loratadine, cetirizine) are preferred because they have lower milk transfer and don’t cause infant sedation.