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Limited evidenceWomen's health

Fennel

Fennel has repeatedly reduced menstrual pain in small randomized trials — meta-analyses find it beats placebo and performs roughly like mefenamic acid — but almost every study is small, short and single-region, so certainty stays low. Its traditional uses for colic and bloating rest on weak evidence, and infant use raises real safety concerns.

What the science says

  • Reduces primary dysmenorrhea pain Moderate evidence

    A 2020 systematic review and meta-analysis of randomized trials found fennel significantly reduced menstrual pain versus placebo and produced relief comparable to mefenamic acid, though it was weaker on some secondary measures. A 2021 meta-analysis reached similar conclusions. Trials typically enrolled 50-100 participants over 1-3 menstrual cycles and were rated low quality; a Cochrane review of supplements for dysmenorrhoea judged herbal evidence overall to be low or very low certainty.

  • Relieves infantile colic Limited evidence

    A Cochrane review of dietary modifications for infantile colic found fennel-containing herbal preparations reduced crying time in small trials but rated the evidence as low quality with unclear safety, and did not recommend routine use. Fennel tea in infants has been linked to reduced milk intake and, in case reports, to estrogenic effects.

  • Eases menopausal symptoms Limited evidence

    Fennel contains anethole, a phytoestrogen, and small randomized trials report reduced hot flushes and improved vaginal atrophy scores over 8-10 weeks. The studies are underpowered, and the phytoestrogenic mechanism is exactly what makes long-term safety in hormone-sensitive conditions uncertain.

  • Relieves bloating and IBS symptoms Limited evidence

    Fennel is a traditional carminative and appears in combination products (for example with curcumin) that have shown benefit in IBS. Fennel alone has not been adequately tested against placebo in IBS.

Dosage & safety

Studied doseDysmenorrhea trials used fennel extract 30-60 mg two to four times daily (or 1-2% fennel essential oil drops), started 2-3 days before menses and continued through the first 3 days of bleeding. Menopause trials used 100 mg twice daily for 8-10 weeks. Culinary use as tea (1-3 g of crushed seed per cup) is a much lower exposure.
SafetyGenerally well tolerated; possible nausea, and photosensitivity or allergic contact reactions in people sensitive to Apiaceae plants (celery, carrot, mugwort). Fennel's estrogenic activity means it should be avoided or discussed with a clinician in hormone-sensitive conditions such as breast, uterine or ovarian cancer and endometriosis, and it may interact with tamoxifen and hormone therapy. Fennel essential oil contains estragole, a rodent carcinogen; the EMA advises against prolonged high-dose use and against use in children under 4 and in pregnancy or breastfeeding. Do not give fennel tea to infants for colic without paediatric advice — it can displace milk intake. May reduce absorption of ciprofloxacin.

Scientific references

Where to buy

Nutricost, Fennel Seed, 240 Capsules (200 mg per Capsule)

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