Fennel

Foeniculum vulgare Mill. belongs to Apiaceae (Umbelliferae), the carrot family, alongside celery, dill, anise, caraway, and coriander. Badgujar and colleagues (BioMed Research International 2014) note that the name must be credited to Philip Miller, who first validly published it in the eighth edition of his Gardeners Dictionary in 1768; Mrs Grieve’s A Modern Herbal (1931) adds that Linnaeus had placed fennel in Anethum, the dill genus, before de Candolle gave it a genus of its own, and that Foeniculum was the Romans’ name for the plant, from the Latin foenum, hay. Badgujar and colleagues describe fennel as originating in the southern Mediterranean and now growing wild across Asia, North America, and Europe. It is an upright, branching perennial up to about 2 m tall, with smooth, bright green stems so glossy they seem polished. The leaves, up to 40 cm long, are cut into thread-like segments about 0.5 mm wide, and their bases sheathe the stem.

Small yellow flowers are borne in large, flat compound umbels with 13 to 20 rays, blooming in July and August (Grieve). The fruits are oblong, about 3–5 mm long and 1.5–2 mm wide, with strong ribs (Badgujar), and split into two halves, which is why pharmacopoeias describe the drug as dry cremocarps and mericarps. EMA, following the European Pharmacopoeia, distinguishes two medicinal varieties. Bitter fennel (var. vulgare) is greenish-brown, brown, or green and must contain at least 40 ml of essential oil per kilogram, with at least 60% anethole and 15% fenchone. Sweet fennel (var. dulce) is pale green or pale yellowish-brown, with at least 20 ml of oil per kilogram and at least 80% anethole. Florence fennel, or finocchio, is var. azoricum (Vella, Molecules 2024), grown for its swollen white leaf bases; Parkinson recorded in 1640 that Italians transplanted and whitened fennel to make it tender (Grieve). Research on the seeds does not describe the vegetable.

The essential oil carries the plant’s character. Trans-anethole (PubChem CID 637563, C10H12O), also the main constituent of anise oil, gives the sweet aniseed taste; fenchone (CID 14525, C10H16O) is camphor-like and bitter. Estragole, or methyl chavicol (CID 8815), has exactly the same formula as anethole and differs only in the position of one double bond in its side chain. EMA’s assessment report notes chemotypes with less than 50% anethole and more than 30% each of fenchone and estragole, and the European Pharmacopoeia caps estragole in bitter fennel oil at 6.0%. The fruits also contain small amounts of bergapten, a furanocoumarin that may cause phototoxicity. Outside its home range fennel can be a serious weed: the California Invasive Plant Council rates it moderately invasive, with dense stands on Santa Cruz Island, around San Francisco Bay, on the Palos Verdes Peninsula, and at Camp Pendleton, where it can drastically alter grassland, coastal scrub, riparian, and wetland communities. It is still unclear whether culinary varieties are invasive.

Fennel is one of the oldest cultivated flavorings. Vella and colleagues (Molecules 2024) write that it was known to the ancient Egyptians, Romans, Indians, and Chinese, that early Sanskrit writings called it madhurika, and that its cultivation in India dates back to at least 2000 BC. To the ancient Greeks fennel represented success; its Greek name was marathon, and the battle of Marathon in 490 BC is said to have been fought in a fennel field. The Romans, too, treated fennel as a symbol of triumph and used its leaves to crown the winners of games. Mrs Grieve offers another explanation of the Greek name, linking it to maraino, to grow thin, and quotes William Coles (1650), who said fennel drinks and broths made those grown fat ‘more gaunt and lank.’ Pliny, she notes, credited fennel with 22 remedies and reported that snakes ate it when shedding their skins and sharpened their sight with its juice, the root of a long belief that fennel strengthens the eyes.

Fennel spread through medieval Europe as food, medicine, and charm. Grieve records it in Anglo-Saxon cookery and medical recipes from before the Norman Conquest and in a Spanish agricultural record of AD 961 that mentions fennel shoots, fennel water, and seed. She says its diffusion in Central Europe was stimulated by Charlemagne, who enjoined its cultivation on the imperial farms, and Badgujar and Vella likewise credit the emperor with encouraging it. Fennel was hung over doors on Midsummer’s Eve, with St. John’s wort, to ward off evil spirits, eaten with salt fish during Lent, and bought by the pound for Edward I’s household. Culpeper wrote that fennel expels wind, that the leaves or seed boiled in barley water increase nurses’ milk, and that the seeds stay hiccups and ‘inclination to sickness.’ Gerard and Parkinson recorded it as a garden and kitchen plant.

By the early twentieth century fennel had settled into a modest medical role. Grieve describes it as an ingredient of compound liquorice powder, added to purgatives to allay their griping, and explains that fennel water, like dill and anise water, was mixed with sodium bicarbonate and syrup to make the domestic gripe water used for infants’ wind. EMA’s assessment report traces fennel through an Italian pharmacopoeia of 1851 and through Chinese medicine, where it is xiaohuixiang, listed in the Pharmacopoeia of the People’s Republic of China for lower abdominal pain with a cold sensation, period pain, and stomach pain with poor appetite, at 3–6 g a day. It also records fennel’s long use to increase breast milk. The EMA committee first adopted European monographs on bitter and sweet fennel fruit in July 2007, revised them in January 2024, and in May 2024 concluded that its monograph on bitter fennel oil could no longer be supported.

Fennel’s digestive reputation rests on laboratory evidence that it relaxes smooth muscle. EMA’s assessment report summarizes studies in which alcoholic extracts and the oil of fennel fruit relaxed isolated guinea pig intestine contracted by acetylcholine, histamine, or carbachol, although once the effect of the alcohol itself was allowed for, only the histamine result was significant. Anethole had a two-sided effect on isolated mouse intestine, stimulating movement at low concentrations and relaxing it at higher ones, and in rats it improved the stomach’s ability to relax and accommodate a meal. Fennel extracts also inhibit a wide range of bacteria and fungi in the test tube, which EMA suggests might contribute to relieving bloating and flatulence. EMA is clear about the limits: clinical trials showing that fennel works as a spasmolytic are limited and preliminary.

Similar muscle-relaxing effects have been shown on the uterus and airways, again mostly outside the body. Ostad and colleagues (Journal of Ethnopharmacology 2001) found that fennel essential oil reduced the strength of contractions induced by oxytocin and prostaglandin E2 in isolated rat uterus, and reduced the frequency of prostaglandin-induced contractions; prostaglandins drive much of the pain of primary dysmenorrhea. For the respiratory tract, EMA cites an infusion of bitter fennel that increased mucociliary transport, the movement of mucus by the tiny hairs lining the airways, by about 12% in isolated frog tissue, and experiments in rabbits in which inhaled fenchone increased the output of respiratory fluid while fenchone and anethole made it thinner. These findings make the traditional uses plausible but do not show that a cup of tea does the same in people.

Two other kinds of activity matter for safety. Anethole shows weak estrogenic activity at high concentrations in laboratory tests, and acetone extracts of fennel had estrogenic effects in rats, although EMA concluded that anethole is not expected to act as an estrogen when fennel is taken as tea at recommended doses. Estragole is the bigger concern. EMA’s 2023 public statement explains that the liver converts it to 1′-hydroxyestragole, whose sulfate conjugate binds to DNA; this pathway causes liver tumors in mice and also operates in human cells in the laboratory, though the effect is dose-dependent and diminishes markedly at low exposures. Using the Carcinogenic Potency Database figure of 51.8 mg/kg a day and a standard international formula, EMA calculated a guidance value of about 0.05 mg of estragole per person per day, the basis for its limits on medicines.

Bloating, wind, and mild cramping: this is fennel’s core traditional use, and EMA accepts both bitter and sweet fennel fruit tea for symptomatic treatment of mild, spasmodic gastrointestinal complaints including bloating and flatulence. The listing is explicitly based on long-standing use: EMA’s assessment report found no clinical studies relevant to the indication and concluded that the medicinal use of fennel fruit is not supported by adequate clinical evidence. That does not mean fennel tea does nothing, only that it has not been properly tested. Roasted or sugar-coated fennel seeds are widely chewed as mukhwas, a mouth freshener, in India and Pakistan (Badgujar), and a warm cup after a heavy meal is a reasonable, low-cost thing to try for occasional fullness and wind.

Irritable bowel syndrome: the best trial used fennel oil in combination. Portincasa and colleagues (Journal of Gastrointestinal and Liver Diseases 2016) randomized 121 people with mild-to-moderate IBS to capsules of curcumin and fennel essential oil or placebo, two capsules twice a day for 30 days. Symptom severity scores fell by an average of 50% with the combination and 26% with placebo, and 25.9% of treated patients became symptom-free, against 6.8% on placebo; quality of life improved too, and the product was well tolerated. Because the capsules also contained curcumin from turmeric, the trial cannot show what fennel contributed. A later open study by Di Ciaula and colleagues (Annals of Gastroenterology 2018) followed 211 patients taking the same product for two months and reported improvements in every IBS subtype, but it had no control group, so placebo effects and natural ups and downs could explain much of the change.

Infant colic: Alexandrovich and colleagues (Alternative Therapies in Health and Medicine 2003) randomized 125 infants aged 2 to 12 weeks to a fennel seed oil emulsion or placebo. Colic resolved, meaning total crying fell below nine hours a week, in 65% (40 of 62) of treated infants and 23.7% (14 of 59) on placebo, and no side effects were reported. Savino and colleagues (Phytotherapy Research 2005) gave 93 breastfed colicky infants a standardized extract of chamomile, fennel, and lemon balm or placebo twice a day for a week; among the 88 who finished, daily crying fell from about 201 to 77 minutes, compared with 199 to 170 minutes on placebo. Cochrane’s review by Biagioli and colleagues (2016) included 18 trials with 1,014 infants, all small and at high risk of bias, and found low-quality evidence that herbal agents shorten crying, with very inconsistent results, and moderate-quality evidence that more infants respond. Its authors concluded that herbal agents cannot be recommended, and EMA does not recommend fennel under age 4.

Period pain: EMA accepts fennel tea as a traditional remedy for minor spasm associated with menstrual periods in adults and adolescents. In a trial by Namavar Jahromi and colleagues (International Journal of Gynecology and Obstetrics 2003), 30 women aged 15 to 24 with moderate-to-severe period pain had one untreated cycle, one with mefenamic acid 250 mg every six hours, and one with a 2% fennel essence, 25 drops every four hours. Both treatments relieved pain compared with the untreated cycle and began working in a similar time, a little over an hour, but mefenamic acid was stronger on the second and third days; five women (16.6%) dropped out because of fennel’s smell. Lee and colleagues (Nutrients 2020) pooled 12 trials: in seven, fennel performed similarly to conventional drugs (502 participants), and against placebo it looked strongly favorable, but with extreme heterogeneity (I² of 98%) and signs that small positive studies were overrepresented. Only three trials assessed side effects. EMA judged the trials too flawed to support well-established use.

Menopause, breast milk, and coughs: Rahimikian and colleagues (Menopause 2017) randomized 90 postmenopausal women aged 45 to 60 in Tehran to soft capsules containing 100 mg of fennel or placebo, two a day for eight weeks, and found significantly better menopause symptom scores with fennel at 4, 8, and 10 weeks; the authors asked for larger trials. EMA’s assessment report notes that most menopause studies of fennel failed to beat placebo and that positive ones were small and short. Fennel is a traditional galactagogue, but Cochrane’s review of oral galactagogues by Foong and colleagues (2020) found that investigators are very uncertain whether fennel improves infant weight, and LactMed notes that no increase in prolactin has been found in nursing mothers using fennel. The expectorant use for coughs with colds is supported only by tradition and laboratory work. Reported antimicrobial, blood-sugar-lowering, and anticancer activities come largely from laboratory and animal studies (Badgujar), and EMA found no other medicinal uses supported by adequate data.

EMA’s 2024 monographs cover only one form, herbal tea made from the dried fruit. For adults and adolescents the single dose is 1.5 g of fennel fruit in 250 ml of boiling water, steeped for 15 minutes, three times a day, a daily total of 4.5 g, for digestive complaints, coughs, or period cramps. For children aged 4 to 12, the digestive and cough dose is 1 g in 100 ml three times a day, for less than a week, and only if the product keeps estragole intake below EMA’s guidance value. Adults should not take it for more than two weeks, and anyone whose symptoms persist or worsen should see a doctor. EMA notes that fennel fruit is usually crushed before use and that crushed fruit gradually loses its volatile oil; tea bags tested 30 days after opening had lost 4–10% of their oil. Mrs Grieve’s older recipe of a teaspoon of bruised seeds to half a pint of boiling water is similar.

How the tea is made changes how much estragole it contains. EMA’s estragole statement reports that infusions made from whole fennel fruits contained about three times less estragole than those made from finely cut material, and that squeezing the tea bag at the end and using broken fruits increased the estragole in the cup. Commercial fennel teas measured in one study contained 241 to 2,058 µg of estragole per liter from tea bags and 9 to 912 µg per liter from diluted instant teas, some of them designed for babies. Three cups a day gave adults an estimated maximum of 10 µg/kg of body weight daily, while estimated exposure in infants reached 51 µg/kg from tea bags, far above EMA’s guidance value of 1.0 µg/kg for children. Short courses, whole rather than finely cut seeds, and not squeezing the bag are sensible ways to keep exposure down.

Trials used forms that are not interchangeable with tea: a fennel seed oil emulsion for infants, a 2% fennel essence given as drops for period pain, 100 mg fennel capsules for menopause, and capsules combining fennel oil with curcumin for IBS. Fennel essential oil is the most concentrated source of estragole, and the European Pharmacopoeia allows up to 6.0% in bitter fennel oil. In May 2024 EMA withdrew its monograph on bitter fennel oil because the estragole intake from traditional expectorant doses posed risks not balanced by the benefits, noting that safer options exist. Do not swallow fennel essential oil or give it to children, and keep it out of their reach. Gripe waters and colic drops vary widely in their ingredients, so read the label. Fennel bulb, leaves, and seeds eaten in ordinary cooking are food, not medicine.