Fennel: a quiet vegetable and a much-studied seed

The strongest number attached to fennel is about crying babies, and it comes from the seed. Pooling three small trials in colicky breastfed infants under six months, fennel oil or seed preparations cut crying by about 72 minutes a day, with a 95% interval running from 18 to 126 minutes. The review did not weigh the safety of giving these preparations to infants. The European Medicines Agency does not recommend fennel seed tea as a medicine for children under 4, for lack of data, nor fennel seed preparations in pregnancy or breastfeeding. We found no trial of the bulb you slice into a salad. For period pain, the Cochrane review had a single trial of 43 women, where fennel against placebo moved pain by 0.34 points on a 10-point scale (95% CI 0.74 lower to 0.06 higher), an interval that includes no effect. For breast milk, another Cochrane review says it is very uncertain whether fennel helps babies gain weight. In 64 overweight women with PCOS, fennel capsules added nothing measurable to a calorie-cut diet for weight or androgen levels. So the research is about seed oils, teas and capsules, and none of it shows a health effect you could expect from eating the vegetable.

The numbers, per 3.5 oz (100 g)

How much is actually in it

• Across 8 USDA samples of raw fennel bulb, vitamin C ran from 9.7 to 19.8 mg and folate from 23 to 50 mcg per 3.5 oz (100 g). — USDA Foundation Foods, Fennel, bulb, raw, 8 samples (n = 8 samples) Evidence E source
  Vitamin C 14.7 mg (9.73-19.8, 16% DV); folate 37.02 mcg (23.11-49.85); calcium 41.3 mg (33.4-52.9) per 3.5 oz (100 g)
• UK tables show boiling Florence fennel drops vitamin C from 5 to 2 mg, folate from 42 to 26 mcg and potassium from 440 to 300 mg per 3.5 oz (100 g). — CoFID 2021, Fennel, Florence, raw (13-241) vs boiled in unsalted water (13-542) (n = 2 records) Evidence E source
  Vitamin C 5 to 2 mg; folate 42 to 26 mcg; potassium 440 to 300 mg per 3.5 oz (100 g)

What a real portion looks like

• The US survey food database weighs one fennel bulb at 235 g and an unspecified amount at 25 g. — FNDDS food code 75109010, Fennel bulb, raw (n = 2 portion measures) Evidence E source
  1 bulb 235 g; quantity not specified 25 g

What cooking and storage change

• A Serbian outbreak of Salmonella Senftenberg in babies was traced to fennel seed in a baby tea that parents made by pouring boiled water over it rather than boiling it. — infants under 12 months in Vojvodina, Serbia, 2007-2008 (n = 14 cases) Evidence D source
  13 of 33 retail samples of the tea positive; strain matched the fennel seed and patients
• In birch-pollen allergy, raw fennel can trigger oral allergy symptoms via PR-10 proteins, which cooking breaks down. — one 12-year-old boy with birch pollen allergy and isolated PR-10 sensitization (n = 1) Evidence D source
  Oral allergy syndrome with raw carrot and fennel; later anaphylaxis to raw carrot, cooked carrot tolerated
• A case series describes four infants who developed methaemoglobinemia, with cyanosis not relieved by oxygen, a few hours after homemade fennel puree. All recovered after methylene blue, and the authors blame improper handling and storage of the vegetable. — four infants seen in an emergency department after homemade fennel puree (n = 4) Evidence D source
  Methaemoglobinemia with cyanosis unresponsive to oxygen; all treated with methylene blue and recovered

What it does to your health

• Pooling 3 trials in colicky breastfed babies under 6 months, fennel preparations cut crying by about 72 minutes a day, with a wide interval. — breastfed or mixed-fed infants under 6 months meeting Wessel colic criteria, 3 RCTs in the fennel subgroup (n = 3 RCTs) Evidence A source
  Mean difference in crying -72.1 min/day (95% CI -126.4 to -17.7)
• In the Cochrane review of supplements for period pain, fennel against placebo moved pain by -0.34 points on a 10-point scale, an interval that includes no effect, from one trial of 43 women. — women with moderate or severe primary dysmenorrhoea, mostly students in Iran; fennel arm one RCT of 43 women (n = 43) Evidence A source
  MD -0.34 points on 0-10 VAS (95% CI -0.74 to 0.06); vs NSAIDs MD -0.70 (-1.81 to 0.41), one RCT of 59
• In an RCT of 64 overweight women with PCOS, adding fennel capsules to a calorie-cut diet gave no extra improvement in weight or androgen measures. — 64 overweight or obese women with PCOS, mean age 28.5, BMI 32.2, 16 per arm in a factorial design (n = 64) Evidence B source
  No significant added effect of fennel on anthropometric or androgen indices

Safety, limits and interactions

• In a double-blind RCT of 60 breastfeeding mothers, a herbal tea with bitter fennel, anise, coriander and fenugreek caused no adverse effects in mothers or babies over 30 days and the first year. — 60 healthy exclusively or fully breastfeeding women without milk insufficiency and their infants (n = 60) Evidence B source
  No adverse effects attributable to the tea; no differences in infant growth or maternal measures (all p>.05)
• Fennel can cross-react with mugwort pollen through an Api g 5-like allergen, part of the mugwort-celery-spice syndrome. — one young man with fennel and mugwort pollen allergy, serum studies (n = 1) Evidence D source
  Fennel extract completely inhibited IgE response to mugwort
• The European Medicines Agency does not recommend fennel seed tea as a medicine for children under 4, for lack of data. From 4 to 12 it advises short use of under one week, and not if the daily estragole intake would exceed 1.0 mcg per kg of body weight. — children under 12 using fennel fruit (seed) herbal teas Evidence E source
  Not recommended under 4 years; ages 4 to 12 only below 1.0 ug estragole per kg body weight a day, under one week
• The European Medicines Agency does not recommend fennel seed preparations in pregnancy or breastfeeding, since their safety is not established, and notes that trans-anethole passes into breast milk. It sets estragole intake for these women below 0.05 mg a day. — pregnant and breastfeeding women using fennel fruit herbal preparations Evidence E source
  Use in pregnancy and lactation not recommended; trans-anethole passes into breast milk; estragole below 0.05 mg per person a day
• A case report describes a 12-month-old girl who developed breast tissue after 2 to 3 teaspoons of fennel tea a day for 6 months. It regressed and was normal within a year of stopping the tea. — one 12-month-old girl given fennel tea daily for 6 months (n = 1) Evidence D source
  Isolated premature thelarche; breast development regressed and normalized within a year after fennel was stopped

What is still unknown

• The Cochrane review of 41 trials on milk-boosting remedies says it is very uncertain whether fennel improves infant weight. — 41 RCTs, 3005 breastfeeding mothers of healthy term infants; fennel among 27 natural galactagogue studies (n = 3005) Evidence A source
  Infant weight data (275 participants across fennel, fenugreek, moringa, mixed tea) could not be pooled; very low certainty

The bottom line

As food, the bulb is a light vegetable with a modest amount of vitamin C. Across 8 USDA samples of raw bulb it averaged 14.7 mg per 3.5 oz (100 g), ranging from 9.7 to 19.8 mg, with folate between 23 and 50 mcg. The UK tables list raw Florence fennel at only 5 mg of vitamin C, a gap of about two thirds that the data do not explain. One bulb weighs about 8.3 oz (235 g). Boiling lowers what is left per 3.5 oz (100 g): in the UK records, vitamin C goes from 5 to 2 mg, folate from 42 to 26 mcg and potassium from 440 to 300 mg. Those are two separate records, and water taken up in the pot dilutes every per-100 g figure. The cautions are about allergy and about seed teas for babies. In one young man, fennel cross-reacted with mugwort pollen through a shared allergen, which places fennel in the mugwort-celery-spice pattern. A 12-year-old boy with birch pollen allergy had mouth symptoms from raw carrot and raw fennel. He tolerated cooked carrot, but cooked fennel was never tested. Both are single cases and say nothing about how common this is. In Serbia in 2007 and 2008, 14 infants caught Salmonella from a baby tea containing fennel seed that parents made by pouring boiled water over it rather than boiling it. 13 of 33 retail samples of that tea tested positive. It was one outbreak from one unregistered seed grower. A trial in 60 breastfeeding mothers found no adverse effects over 30 days and the first year from a tea mixing bitter fennel, anise, coriander and fenugreek. It was small and too short on numbers to catch rare harms, and it does not override the agency advice below. The limits come from the European Medicines Agency and concern medicinal teas and preparations of the seed, not the bulb. It does not recommend fennel seed tea for children under 4, for lack of data. From 4 to 12 it advises use for under one week, and not if the daily estragole intake would exceed 1.0 mcg per kg of body weight. Estragole is the seed compound behind these limits. In pregnancy and breastfeeding the agency does not recommend fennel seed preparations, because their safety is not established, notes that trans-anethole passes into breast milk, and keeps estragole below 0.05 mg a day. Two case reports in babies sit alongside this. A 12-month-old girl given 2 to 3 teaspoons of fennel tea a day for 6 months developed breast tissue, which was normal again within a year of stopping the tea. Four infants developed methaemoglobinemia, with blue skin not relieved by oxygen, a few hours after homemade fennel puree, and all recovered after treatment. The authors blame improper handling and storage of the vegetable. Both are case reports and do not show how often this happens. We have no card on drug interactions for fennel, and that silence is not an all-clear.

Sources

1. fenl-r2-09 · USDA FoodData Central Foundation Foods, dataset
   Fennel, bulb, raw (FDC 2747655, Foundation, release 2026-04-30): vitamin C 14.7 mg, range 9.73-19.8, n=8; folate 37.02 ug, range 23.11-49.85, n=8; calcium 41.3 mg, range 33.4-52.9, n=8
   USDA FoodData Central, Foundation Foods, FDC 2747655 · checked 2026-09-28
2. fenl-r2-10 · CoFID 2021, dataset
   Fennel, Florence, raw (CoFID 13-241): vitamin C 5 mg, folate 42 ug, potassium 440 mg per 100 g; Fennel, Florence, boiled in unsalted water (CoFID 13-542): vitamin C 2 mg, folate 26 ug, potassium 300 mg per 100 g
   CoFID 2021, Public Health England / OGL v3.0, codes 13-241 and 13-542 · checked 2026-09-28
3. fenl-r2-11 · USDA FNDDS 2021-2023, dataset
   Fennel bulb, raw (FNDDS 75109010, FDC 2709779, WWEIA Other vegetables and combinations): 1 fennel bulb 235 g; Quantity not specified 25 g
   USDA FNDDS 2021-2023, food code 75109010 (FDC 2709779) · checked 2026-09-28
4. fenl-r2-06 · outbreak investigation
   Before feeding it to the infants, the parents had not heated the tea until it boiled, but rather had poured boiled water over the tea. After obtaining that information, we tested 33 samples of the incriminated brand of tea from public grocery stores and supermarkets; 13 samples were positive for S. Senftenberg. ... In the fennel seed sample, S. Senftenberg was identified.
   Emerg Infect Dis, 2010 · checked 2026-09-28
5. fenl-r2-08 · case report
   Previously, he experienced only oral allergy syndrome with raw carrot and fennel. ... The reaction occurred during birch pollen season, and the patient tolerated cooked carrot, consistent with PR-10's thermal instability.
   Pediatr Allergy Immunol Pulmonol, 2025 · checked 2026-09-28
6. hf2-fenl-3 · case series
   Four cases of methemoglobinemia (MetHb) are presented here. Each patient ate homemade fennel purée a few hours prior to arriving to the emergency department with cyanosis unresponsive to oxygen, in the absence of suspected cyanotic heart disease. All 4 patients received treatment with methylene blue and fully recovered. Our cases emphasize that improper handling and storing of vegetables can lead to life-threatening MetHb.
   Pediatr Emerg Care, 2005 · checked 2026-10-08
7. fenl-r2-01 · meta-analysis of randomized trials
   The 3 studies included for subgroup meta-analysis on preparations containing fennel suggest it to be effective, with an overall mean difference of -72.1 min/day (95% CI -126.4 to -17.7, P < 0.001).
   J Pediatr Gastroenterol Nutr, 2016 · checked 2026-09-28
8. fenl-r2-02 · Cochrane systematic review
   There was no consistent evidence of effectiveness for dill (MD -1.15 points, 95% CI -2.22 to -0.08, one RCT, 46 women), guava (MD 0.59, 95% CI -0.13 to 1.31; one RCT, 151 women); one RCT, 73 women), or fennel (MD -0.34 points, 95% CI -0.74 to 0.06; one RCT, 43 women).
   Cochrane Database Syst Rev, 2016 · checked 2026-09-28
9. fenl-r2-05 · randomized controlled trial
   CONCLUSION: A hypocaloric high-protein diet along with fennel supplementation could not provide additional improvements in anthropometric and androgen indices among PCOS women.
   Complement Ther Med, 2021 · checked 2026-09-28
10. fenl-r2-04 · randomized controlled trial
    RESULTS: No adverse effects attributable to the interventions were reported at any time point.
    J Hum Lact, 2019 · checked 2026-09-28
11. fenl-r2-07 · case report with laboratory analysis
    Inhibition experiments showed a high degree of cross-reactivity between this fennel allergen and the homologous mugwort pollen allergen.
    Eur Ann Allergy Clin Immunol, 2013 · checked 2026-09-28
12. hf2-fenl-1 · EU herbal monograph
    The use in children between 4 and 12 years of age is not recommended if the daily intake of estragole exceeds the guidance value of 1.0 µg/kg bw, unless justified by a risk assessment based on adequate safety data (see section 5.3 ‘Preclinical safety data’). The use is not recommended in children under 4 years of age due to lack of adequate data. ... For short-term use in mild transitory symptoms only (less than one week).
    European Medicines Agency, HMPC, European Union herbal monograph on Foeniculum vulgare Miller subsp. vulgare var. vulgare, fructus, Revision 1 (EMA/HMPC/372841/2016), 2024 · checked 2026-10-08
13. hf2-fenl-2 · EU herbal monograph
    Safety during pregnancy and lactation has not been established ... In the absence of sufficient data, the use during pregnancy and lactation is not recommended. There is evidence that trans-anethole is excreted in human breast milk. ... In pregnant and breast-feeding women, the daily intake of estragole has to be below 0.05 mg/person per day.
    European Medicines Agency, HMPC, European Union herbal monograph on Foeniculum vulgare Miller subsp. vulgare var. vulgare, fructus, Revision 1 (EMA/HMPC/372841/2016), 2024 · checked 2026-10-08
14. hf2-fenl-4 · case report
    Here, we report a case of a 12-month-old girl who showed breast development that became apparent in the last 3 months. Her medical history revealed that she was given two to three teaspoons of fennel tea by her mother every day for restlessness for the last 6 months. ... On follow-up, after cessation of fennel consumption, the breast development of the patient regressed gradually and became normal by the end of 1 year.
    J Pediatr Endocrinol Metab, 2014 (Okdemir et al.) · checked 2026-10-08
15. fenl-r2-03 · Cochrane systematic review
    Subgroup analysis shows we are very uncertain whether fennel or fenugreek improves infant weight, whereas moringa and mixed botanical tea may increase infant weight compared to placebo.
    Cochrane Database Syst Rev, 2020 · checked 2026-09-28