Fennel

Fennel was prized by the ancient Romans for its succulent shoots and aromatic seed, and was said to promote longevity and to give strength and courage ^{45Reference 45A Modern Herbal · 1931Fennel. https://www.botanical.com/botanical/mgmh/f/fennel01.htmlView study →}.

Pliny and later herbalists recommended it for strengthening the sight ^{45Reference 45A Modern Herbal · 1931Fennel. https://www.botanical.com/botanical/mgmh/f/fennel01.htmlView study →}.

It was also a staple culinary herb, and its pairing with fish survives today ^{45Reference 45A Modern Herbal · 1931Fennel. https://www.botanical.com/botanical/mgmh/f/fennel01.htmlView study →}. It was used both to improve appetite in the underweight and to reduce it in the overweight ^{44Reference 44Bone et al. · 2013Principles and Practice of Phytotherapy (2nd ed.)}.

Fennel seeds have been recovered from Egyptian tombs ^{44Reference 44Bone et al. · 2013Principles and Practice of Phytotherapy (2nd ed.)}. The plant also has a place in Ayurvedic, Unani, Siddha and Iranian traditional medicine ^{11,46Reference 11Badgujar et al. · 2014ReviewFoeniculum vulgare Mill: a review of its botany, phytochemistry, pharmacology and toxicology — reviewView study →Reference 46Rahimi et al. · 2013Medicinal properties of Foeniculum vulgare Mill. in traditional Iranian medicine and modern phytotherapy}.

In Chinese medicine fennel is xiao hui xiang, classified as pungent in taste and warm in nature, entering the Liver, Kidney, Spleen and Stomach channels ^{48Reference 48Wu · 2005An Illustrated Chinese Materia Medica}. It is used to expel cold, relieve pain and regulate the flow of Qi in the stomach ^{48Reference 48Wu · 2005An Illustrated Chinese Materia Medica}.

It is given for cold conditions of the lower abdomen, including cold hernia-type pain, and for cold-stomach patterns with abdominal pain, indigestion, weak appetite and vomiting ^{44,47Reference 44Bone et al. · 2013Principles and Practice of Phytotherapy (2nd ed.)Reference 47Bone · 2003A Clinical Guide to Blending Liquid Herbs}.

Three meta-analyses of randomised trials found fennel reduced menstrual pain about as well as conventional painkillers. In the first, seven trials comparing fennel with drug therapy found no difference in pain (SMD 0.07, 95% CI −0.08 to 0.21), and fennel beat placebo, though the placebo comparison was highly heterogeneous (I²=98%) and an excess of small favourable studies suggested publication bias ^{1Reference 1Lee et al. · 2020Meta-analysisFennel for reducing pain in primary dysmenorrhea — systematic review and meta-analysis of RCTsView study →}.

A second meta-analysis found fennel significantly better than placebo (SMD −0.632, seven trials) and no different from mefenamic acid (six trials) ^{2Reference 2Shahrahmani et al. · 2021Meta-analysisEffect of fennel on primary dysmenorrhea — systematic review and meta-analysisView study →}. A third, pooling trials of cinnamon, fennel and ginger, also found fennel better than placebo ^{3Reference 3Xu et al. · 2020Meta-analysisEfficacy of herbal medicine (cinnamon/fennel/ginger) for primary dysmenorrhea — systematic review and meta-analysis of RCTsView study →}.

In an early head-to-head trial in 110 schoolgirls (mean age 13), 80% on fennel extract and 73% on mefenamic acid reported complete or partial pain relief ^{5Reference 5Modaress Nejad et al. · 2006RCTComparison of fennel and mefenamic acid on pain intensity in dysmenorrhoea — RCTView study →}. A 3-month trial of fennel capsules in 80 students reduced nausea, weakness and the length of the period against an untreated control group ^{6Reference 6Ghodsi et al. · 2014RCTEffect of fennel on pain quality, symptoms, and menstrual duration in primary dysmenorrhea — RCTView study →}.

The Cochrane review could analyse only one fennel-versus-placebo trial (43 women), which found no significant benefit (MD −0.34 points). It rated all the dysmenorrhoea supplement evidence low or very low quality, and 22 of its 27 trials came from Iran ^{4Reference 4Pattanittum et al. · 2016Systematic reviewDietary supplements for dysmenorrhoea — Cochrane systematic reviewView study →}.

In isolated rat uterus, fennel essential oil weakened contractions induced by oxytocin and by prostaglandin E2 ^{32Reference 32Ostad et al. · 2001In vitroThe effect of fennel essential oil on uterine contraction as a model for dysmenorrhea — in vitro (isolated rat uterus)View study →}.

Gap: the positive meta-analyses draw almost entirely on small Iranian trials using different preparations and doses; a large, standardized, multi-region RCT — and independent replication outside a single research culture — is missing.

A randomized placebo-controlled trial in 125 infants aged 2 to 12 weeks found that a fennel seed-oil emulsion eliminated colic (by Wessel criteria) in 65% of treated infants versus 24% on placebo. That is an absolute risk reduction of 41% and a number-needed-to-treat of 2, with no reported side effects ^{12Reference 12Alexandrovich et al. · 2003RCTThe effect of fennel seed oil emulsion in infantile colic — randomised placebo-controlled studyView study →}.

A systematic review of interventions for breast-fed infants pooled three fennel-containing preparations and found a mean reduction of 72 minutes of crying per day versus control ^{13Reference 13Harb et al. · 2016Systematic reviewInfant colic — what works: a systematic review of interventions for breast-fed infantsView study →}.

Fennel seed oil reduces intestinal spasm, the proposed mechanism ^{12Reference 12Alexandrovich et al. · 2003RCTThe effect of fennel seed oil emulsion in infantile colic — randomised placebo-controlled studyView study →}.

Gap: the trials use specific seed-oil emulsions and combination products (often with other Apiaceae), not the fennel-water or tea a parent would brew; the estragole content of infant fennel decoctions has also drawn safety scrutiny ^{14Reference 14Gori et al. · 2012ReviewCan estragole in fennel seed decoctions really be considered a danger for human health? — safety reviewView study →}.

A 2021 systematic review and meta-analysis identified seven RCTs of fennel in menopausal women. The two placebo-controlled trials pooled in favour of fennel for overall menopausal symptoms (SMD −1.32, 95% CI −1.76 to −0.87), with no serious adverse events ^{15Reference 15Lee et al. · 2021Meta-analysisFennel for the management of menopausal women’s health — systematic review and meta-analysisView study →}.

A triple-blind RCT (n=90, 100 mg twice daily, 8 weeks) significantly lowered Menopause Rating Scale scores ^{16Reference 16Rahimikian et al. · 2017RCTEffect of fennel on menopausal symptoms in postmenopausal women — randomised, triple-blind, placebo-controlled trialView study →}. In a 60-woman trial, a 5% fennel vaginal cream improved the vaginal maturation index and pH over 8 weeks ^{17Reference 17Yaralizadeh et al. · 2016RCTEffect of fennel vaginal cream on vaginal atrophy in postmenopausal women — double-blind RCTView study →}.

Trials of fennel for sleep quality ^{19Reference 19Afiat et al. · 2018RCTEffect of fennel on sleep quality of menopausal women — double-blind RCTView study →}, blood lipids ^{20Reference 20Afiat et al. · 2018RCTThe effect of short-term treatment with fennel on lipid profile in postmenopausal women — RCTView study →} and depression and anxiety ^{18Reference 18Ghazanfarpour et al. · 2018RCTEffect of fennel on depression and anxiety in postmenopausal women — double-blind RCTView study →} found no overall effect; the anxiety and depression benefit appeared only in the subgroup that had a disorder.

The effect is attributed to fennel’s estrogen-like activity, which has been linked to anethole and its polymers ^{31Reference 31Albert-Puleo · 1980ReviewFennel and anise as estrogenic agents — reviewView study →}.

In a 2025 organ-bath study, freshly brewed fennel tea relaxed guinea-pig fundus and corpus while strengthening contractions in the antrum. The relaxation was blocked by an inhibitor of store-operated calcium entry and did not depend on nerves ^{7Reference 7Annahazi et al. · 2025Fennel tea has a region-specific effect on the motility of the stomach — ex vivoView study →}.

In rodents, anethole restored delayed gastric emptying and improved gastric accommodation ^{8Reference 8Asano et al. · 2016AnimalAnethole restores delayed gastric emptying and impaired gastric accommodation in rodents — animal modelView study →}.

Direct human trials of fennel alone for dyspepsia are scarce. A placebo-controlled RCT of curcumin plus fennel essential oil in 121 patients with irritable bowel syndrome improved symptom severity and quality of life over 30 days ^{9Reference 9Portincasa et al. · 2016RCTCurcumin and fennel essential oil improve symptoms and quality of life in IBS — RCTView study →}. An open, uncontrolled study of the same combination in 211 patients reported similar improvements over two months ^{10Reference 10Di Ciaula et al. · 2018RCTBio-optimized turmeric and essential fennel oil on quality of life in IBS — RCTView study →}. Neither can separate fennel’s contribution from curcumin’s.

Aqueous fennel extract also raised gastric acid and digestive-enzyme secretion in rats ^{11Reference 11Badgujar et al. · 2014ReviewFoeniculum vulgare Mill: a review of its botany, phytochemistry, pharmacology and toxicology — reviewView study →}.

Gap: the human evidence is combination-product data; there is no modern placebo-controlled RCT of standardized fennel monotherapy for functional dyspepsia or bloating.

Two small placebo-controlled RCTs support topical fennel for idiopathic hirsutism, excess hair in women with normal androgen levels. In a double-blind study in 38 patients, creams containing 1% and 2% fennel fruit extract reduced hair diameter by 7.8% and 18.3%, against −0.5% for placebo ^{24Reference 24Javidnia et al. · 2003RCTAntihirsutism activity of fennel extract — double-blind placebo-controlled studyView study →}.

A second trial of 3% fennel gel in 44 women reduced facial-hair thickness from ~98 to ~76 µm over 24 weeks (p<0.001), with only mild local itching or irritation that did not differ significantly from placebo ^{25Reference 25Akha et al. · 2014RCTEffect of fennel gel 3% in decreasing hair thickness in idiopathic mild-to-moderate hirsutism — RCTView study →}.

The proposed basis is fennel’s estrogen-like activity acting on androgen metabolism in the skin ^{24Reference 24Javidnia et al. · 2003RCTAntihirsutism activity of fennel extract — double-blind placebo-controlled studyView study →}.

Both trials are small, single-centre and short, and they test a cream or gel on the skin.

Fennel fruit methanolic extract, given by mouth at 200 mg/kg, reduced acute and subacute inflammation and type IV allergic reactions in animals, and had a central pain-relieving effect ^{21Reference 21Choi et al. · 2004AnimalAntiinflammatory, analgesic and antioxidant activities of the fruit of Foeniculum vulgare — animal modelView study →}.

Of four coumarins isolated from the fruit, imperatorin was the most active. It suppressed IL-6 and TNF-α in macrophages through the NF-κB pathway, and reduced skin inflammation on mouse ears ^{22Reference 22Yang et al. · 2015In vitroAnti-inflammatory and antioxidant effects of coumarins isolated from Foeniculum vulgare — in vitro / mouseView study →}. Fennel essential oils also inhibited 5-lipoxygenase in vitro ^{23Reference 23Miguel et al. · 2010In vitroFoeniculum vulgare essential oils: chemical composition, antioxidant and antimicrobial activities — in vitroView study →}.

No human trial has tested oral fennel as an anti-inflammatory.

Fennel essential oils scavenge free radicals in vitro, and oils from the aerial parts did so better than fruit oils ^{23Reference 23Miguel et al. · 2010In vitroFoeniculum vulgare essential oils: chemical composition, antioxidant and antimicrobial activities — in vitroView study →}. At higher concentrations the oils acted as pro-oxidants in one assay ^{23Reference 23Miguel et al. · 2010In vitroFoeniculum vulgare essential oils: chemical composition, antioxidant and antimicrobial activities — in vitroView study →}. The coumarins isolated from the fruit are also strong radical scavengers ^{22Reference 22Yang et al. · 2015In vitroAnti-inflammatory and antioxidant effects of coumarins isolated from Foeniculum vulgare — in vitro / mouseView study →}.

In rats, fennel fruit extract raised SOD and catalase and lowered lipid peroxidation ^{21Reference 21Choi et al. · 2004AnimalAntiinflammatory, analgesic and antioxidant activities of the fruit of Foeniculum vulgare — animal modelView study →}, and seed and sprout extracts restored antioxidant enzymes in a chemical kidney-injury model ^{28Reference 28Barakat et al. · 2023AnimalNephroprotective effect of fennel seeds and sprouts on CCl₄-induced nephrotoxicity in rats — animal modelView study →}.

No human antioxidant-biomarker trial exists.

Fennel essential oil protected rat liver against carbon tetrachloride-induced injury, a standard chemical liver-injury model ^{26Reference 26Ozbek et al. · 2003AnimalHepatoprotective effect of Foeniculum vulgare essential oil — rat in vivoView study →}. d-Limonene and β-myrcene in the oil have been proposed as the active constituents ^{11Reference 11Badgujar et al. · 2014ReviewFoeniculum vulgare Mill: a review of its botany, phytochemistry, pharmacology and toxicology — reviewView study →}.

In mice, an extract of fennel root bark, a traditional Uyghur medicine, reduced carbon tetrachloride-induced liver fibrosis ^{27Reference 27Zhang et al. · 2017AnimalProtective effects of fennel root bark extract against CCl₄-induced hepatic fibrosis in mice — animal modelView study →}. Seed and sprout extracts showed a similar protective pattern in the rat kidney ^{28Reference 28Barakat et al. · 2023AnimalNephroprotective effect of fennel seeds and sprouts on CCl₄-induced nephrotoxicity in rats — animal modelView study →}.

There is no evidence in human liver disease.

Fennel has long been used to increase breast-milk flow ^{31Reference 31Albert-Puleo · 1980ReviewFennel and anise as estrogenic agents — reviewView study →}. Dopamine inhibits the release of prolactin, the milk-producing hormone, and anethole has been proposed to compete with dopamine at its receptors; its dimers dianethole and photoanethole may be the active agents ^{11,31Reference 11Badgujar et al. · 2014ReviewFoeniculum vulgare Mill: a review of its botany, phytochemistry, pharmacology and toxicology — reviewView study →Reference 31Albert-Puleo · 1980ReviewFennel and anise as estrogenic agents — reviewView study →}.

Gap: high-quality human evidence for fennel alone increasing milk volume is thin — most data are traditional, mechanistic, or from mixed herbal teas — and the same estrogenic constituents raise unresolved questions about infant exposure through milk.

Results for the essential oil are mixed. Seed oils from three regions of Iran inhibited bacterial growth in disc-diffusion tests ^{33Reference 33Ghasemian et al. · 2020In vitroChemical composition and antimicrobial and cytotoxic activities of Foeniculum vulgare essential oils — in vitroView study →}, while commercial fennel oils in another study showed very low antimicrobial activity ^{23Reference 23Miguel et al. · 2010In vitroFoeniculum vulgare essential oils: chemical composition, antioxidant and antimicrobial activities — in vitroView study →}. Reviews report activity of fennel extracts and oils against bacteria, fungi and viruses ^{11Reference 11Badgujar et al. · 2014ReviewFoeniculum vulgare Mill: a review of its botany, phytochemistry, pharmacology and toxicology — reviewView study →}.

All of it is in vitro, and fennel has not been tested as a systemic antimicrobial in people.

Fennel and anise have been used as estrogenic agents for millennia. They were reputed to increase milk secretion, promote menstruation, ease childbirth and increase libido ^{31Reference 31Albert-Puleo · 1980ReviewFennel and anise as estrogenic agents — reviewView study →}.

Anethole, the main constituent of the oil, was long considered the active estrogenic agent. Later research suggests its polymers, dianethole and photoanethole, are responsible ^{31Reference 31Albert-Puleo · 1980ReviewFennel and anise as estrogenic agents — reviewView study →}.

This activity is the proposed basis of the menopause, period-pain, hirsutism and galactagogue uses. Its size in humans has not been measured.

A methanolic extract of the whole plant, given to mice for eight days, inhibited acetylcholinesterase and reversed scopolamine-induced and age-related memory deficits ^{29Reference 29Joshi et al. · 2006AnimalCholinergic basis of the memory-strengthening effect of Foeniculum vulgare — mouse in vivoView study →}. In a 2025 study, fennel essential oil improved memory in zebrafish given scopolamine, normalised acetylcholinesterase activity and reduced oxidative stress in the brain ^{30Reference 30Brinza et al. · 2025AnimalFoeniculum vulgare mitigates scopolamine-induced cognitive deficits in zebrafish — animal modelView study →}.

Gap: entirely animal; there is no human cognition trial.

An aqueous leaf extract lowered blood glucose in normal and streptozotocin-diabetic rats ^{36Reference 36El-Ouady et al. · 2020AnimalAntihyperglycemic effect of the aqueous extract of Foeniculum vulgare in diabetic rats — animal modelView study →}, and a methanolic extract lowered glucose, triglycerides and cholesterol in diabetic rats over 28 days ^{37Reference 37Bougrine et al. · 2024AnimalHypoglycemic activity of methanolic extract of Foeniculum vulgare in diabetic Wistar rats — animal modelView study →}.

There is no human data.

In a placebo-controlled crossover trial in nine overweight Korean women, a single serving of fennel tea reduced hunger and prospective food intake ratings, but did not change the amount eaten at the following buffet ^{38Reference 38Bae et al. · 2015RCTFennel and fenugreek tea drinking suppresses subjective short-term appetite in overweight women — randomised crossover trialView study →}.

In 64 overweight women with polycystic ovary syndrome, adding fennel to a calorie-restricted diet produced no additional improvement in weight or androgen measures ^{39Reference 39Nadjarzadeh et al. · 2021RCTHypocaloric high-protein diet supplemented with fennel on androgenic/anthropometric indices in overweight women with PCOS — RCTView study →}.

Traditional sources give fennel both to stimulate appetite in the underweight and to reduce it in the overweight ^{44Reference 44Bone et al. · 2013Principles and Practice of Phytotherapy (2nd ed.)}.

Ethanol extracts of fennel seed reduced the growth of liver-cancer and lung-cancer cells in vitro, and slowed the growth of the same tumours transplanted into nude mice ^{34,35Reference 34Ke et al. · 2021In vitroFoeniculum vulgare seed extract exerts anti-cancer effects on hepatocellular carcinoma — in vitroView study →Reference 35Ke et al. · 2021In vitroFoeniculum vulgare seed extract induces apoptosis in lung cancer cells partly via Bcl-2 down-regulation — in vitroView study →}. Seed essential oils were cytotoxic to MCF-7 breast-cancer cells, raising Bax and lowering Bcl-2 ^{33Reference 33Ghasemian et al. · 2020In vitroChemical composition and antimicrobial and cytotoxic activities of Foeniculum vulgare essential oils — in vitroView study →}.

Gap: no human data, and the in-vivo work is limited to two mouse xenograft studies.

Tolerability. Human trials of seed, capsule and topical preparations report no serious adverse events ^{15,16Reference 15Lee et al. · 2021Meta-analysisFennel for the management of menopausal women’s health — systematic review and meta-analysisView study →Reference 16Rahimikian et al. · 2017RCTEffect of fennel on menopausal symptoms in postmenopausal women — randomised, triple-blind, placebo-controlled trialView study →}.

Estragole. Estragole, a constituent of the essential oil, is suspected of causing liver tumours in susceptible strains of mice ^{41Reference 41Villarini et al. · 2014In vitroCytotoxic, genotoxic and apoptosis-inducing effects of estragole isolated from fennel — in vitroView study →}. In human liver cells in vitro it did not damage DNA or trigger cell death ^{41Reference 41Villarini et al. · 2014In vitroCytotoxic, genotoxic and apoptosis-inducing effects of estragole isolated from fennel — in vitroView study →}. An analysis of fennel seed decoctions argues that other substances in the tea inactivate much of the estragole, so the risk from tea and seed is lower than from the pure compound ^{14Reference 14Gori et al. · 2012ReviewCan estragole in fennel seed decoctions really be considered a danger for human health? — safety reviewView study →}. Estragole in infant fennel decoctions has drawn particular safety concern ^{14Reference 14Gori et al. · 2012ReviewCan estragole in fennel seed decoctions really be considered a danger for human health? — safety reviewView study →}.

Estrogen-like activity. Fennel has traditional estrogenic uses and estrogen-like activity ^{11,31Reference 11Badgujar et al. · 2014ReviewFoeniculum vulgare Mill: a review of its botany, phytochemistry, pharmacology and toxicology — reviewView study →Reference 31Albert-Puleo · 1980ReviewFennel and anise as estrogenic agents — reviewView study →}, so care is sensible in hormone-sensitive conditions.

Drug metabolism. In laboratory tests, fennel extract inhibited CYP2D6 and CYP3A4; this has not been tested in people ^{55Reference 55Langhammer et al. · 2014In vitroIn vitro inhibition of human CYP1A2, CYP2D6, and CYP3A4 by six herbs commonly used in pregnancy — in vitroView study →}. Part of the CYP3A4 effect comes from bergapten (5-methoxypsoralen), which inactivated the enzyme in human liver microsomes ^{57Reference 57Subehan et al. · 2007In vitroInhibition on human liver cytochrome P450 3A4 by constituents of fennel (Foeniculum vulgare): identification and characterization of a mechanism-based inactivator — in vitroView study →}.

Ciprofloxacin. In rats, fennel extract given with the antibiotic ciprofloxacin cut its peak blood level by 83%. The authors traced this to minerals in the extract binding the drug, not to liver enzymes ^{56Reference 56Zhu et al. · 1999AnimalEffect of oral administration of fennel (Foeniculum vulgare) on ciprofloxacin absorption and disposition in the rat — animal modelView study →}.

Allergy. Rare IgE-mediated allergy occurs, chiefly in people sensitised to other members of the carrot family (celery, carrot, anise) ^{40Reference 40Zahi et al. · 2025ReviewCardiovascular effects, phytochemistry, drug interactions, and safety profile of Foeniculum vulgare — comprehensive reviewView study →}.

Duration. Commission E recommends that medicinal doses of fennel not be taken for longer than several weeks, though culinary amounts are widely regarded as safe long-term ^{44Reference 44Bone et al. · 2013Principles and Practice of Phytotherapy (2nd ed.)}.

Toxicity. The oral LD50 of fennel essential oil in rats is about 1.3 g/kg ^{11,32Reference 11Badgujar et al. · 2014ReviewFoeniculum vulgare Mill: a review of its botany, phytochemistry, pharmacology and toxicology — reviewView study →Reference 32Ostad et al. · 2001In vitroThe effect of fennel essential oil on uterine contraction as a model for dysmenorrhea — in vitro (isolated rat uterus)View study →}.

Seed quality. Some lower-grade fennel seed on the market has already been steam-distilled, removing most of the volatile oil. Such seed looks darker, smells faint and sinks at once in water. It is occasionally re-coloured to look fresh, and the colour rubs off on the fingers ^{45Reference 45A Modern Herbal · 1931Fennel. https://www.botanical.com/botanical/mgmh/f/fennel01.htmlView study →}.