Fennel is a plant in the carrot family whose seeds (botanically, fruits) are used as a spice and in herbal medicine. As a supplement it is studied mainly for period pain and menopause symptoms.
Also known as: Foeniculum vulgare, fennel seed, sweet fennel, fennel fruit.
A menopause trial used two 100 mg soft capsules a day for 8 weeks (Rahimikian 2017). Period-pain trials used various fennel preparations (Lee 2020). These are study protocols, not a recommendation.
The NIH Dietary Supplement Label Database lists fennel products such as Fennel 450 mg from Solaray as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling. In the EU, the European Medicines Agency's herbal committee has adopted an EU herbal monograph on sweet fennel fruit (revised 2024); EMA lists its therapeutic areas as cough and cold and urinary tract and genital disorders (EMA).
What studies show: a 2020 review of 12 trials found fennel eased period pain more than placebo and about as well as standard pain drugs. The authors noted signs of publication bias, with many small studies showing good results (Lee 2020). In a trial in 90 postmenopausal women in Tehran, 8 weeks of fennel capsules lowered menopause symptom scores more than placebo (Rahimikian 2017).
Safety: in the 2020 review only 3 studies looked for side effects, and one reported minor ones (Lee 2020). The sources here do not describe drug interactions.
We could not find a single published half-life or time to peak for fennel that applies to a supplement dose, so we do not list one. Plant extracts are mixtures of many compounds, so no single figure would describe them.
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