Fennel
Fennel

Background

Fennel (Foeniculum vulgare) is an herb with yellow flowers. The dried seeds are used in food. The dried seeds and oil are also used as medicine.

Fennel is native to the Mediterranean, but is now found throughout the world. As medicine, it might relax the colon, and also appears to contain an ingredient that may act like estrogen in the body. As a spice, fennel has an anise-like taste.

People use fennel for menstrual cramps. It is also used for excessive crying in infants (colic), indigestion, and symptoms of menopause, but there is no good scientific evidence to support most of these uses.
When taken by mouth: Fennel is commonly consumed in foods. It is possibly safe when used as medicine at appropriate doses for a short period of time. There isn't enough reliable information to know whether fennel is safe when used long-term. Although rare, side effects might include stomach upset and seizures.

When applied to the skin: Fennel is possibly safe. Fennel can make skin extra sensitive to the sun and make it easier to get a sunburn. Wear sunblock if you are light-skinned.

Special Precautions & Warnings:

Pregnancy: Fennel is possibly unsafe to use when pregnant. Regularly using fennel has been linked to preterm birth.

Breast-feeding: Fennel is possibly unsafe. There are some reports of breast-feeding infants with damage to their nervous systems after they were exposed to herbal tea containing fennel through breastmilk.

Children: Fennel is possibly safe when used at appropriate doses for up to one week in young infants with colic.

Allergy to celery, carrot or mugwort: Fennel might cause an allergic reaction in people who are sensitive to these plants.

Bleeding disorders: Fennel might slow blood clotting. Taking fennel might increase the risk of bleeding or bruising in people with bleeding disorders.

Hormone-sensitive condition such as breast cancer, uterine cancer, ovarian cancer, endometriosis, or uterine fibroids: Fennel might act like estrogen. If you have any condition that might be made worse by estrogen, do not use fennel.

Effectiveness

NatMed Pro rates effectiveness based on scientific evidence according to the following scale: Effective, Likely Effective, Possibly Effective, Possibly Ineffective, Likely Ineffective, Ineffective, and Insufficient Evidence to Rate.
Likely effective Effectiveness definitions
Possibly effective Effectiveness definitions
  • Menstrual cramps (dysmenorrhea). Taking fennel oil or fennel extract by mouth seems to improve pain in people with menstrual cramps. Its effect on pain might be similar to drugs such as ibuprofen or mefenamic acid.
There is interest in using fennel for a number of other purposes, but there isn't enough reliable information to say whether it might be helpful.
Likely ineffective Effectiveness definitions
Possibly ineffective Effectiveness definitions
Insufficient evidence Effectiveness definitions

Dosing & administration

Fennel is available in many different types of products, including essential oils, seed extracts, seed powders, teas, and creams. There isn't enough reliable information to know what an appropriate dose of fennel might be. Keep in mind that natural products are not always necessarily safe and dosages can be important. Be sure to follow relevant directions on product labels and consult a healthcare professional before using.

Interactions with pharmaceuticals

Birth control pills (Contraceptive drugs)

Interaction Rating=Moderate Be cautious with this combination.

Some birth control pills contain estrogen. Large amounts of fennel might affect estrogen levels in the body. Taking fennel along with birth control pills might decrease the effects of birth control pills. If you take birth control pills along with fennel, use an additional form of birth control such as a condom.

Ciprofloxacin (Cipro)

Interaction Rating=Moderate Be cautious with this combination.

Ciprofloxacin is an antibiotic. Fennel might decrease how much ciprofloxacin the body absorbs. Taking fennel along with ciprofloxacin might decrease the effects of ciprofloxacin. To avoid this interaction, take fennel at least one hour after ciprofloxacin.

Estrogens

Interaction Rating=Moderate Be cautious with this combination.

Large amounts of fennel might have some of the same effects as estrogen. Taking fennel along with estrogen might decrease the effects of estrogen.

Medications changed by the liver (Cytochrome P450 3A4 (CYP3A4) substrates)

Interaction Rating=Moderate Be cautious with this combination.

Some medications are changed and broken down by the liver. Fennel might change how quickly the liver breaks down these medications. This could change the effects and side effects of these medications.

Medications that slow blood clotting (Anticoagulant / Antiplatelet drugs)

Interaction Rating=Moderate Be cautious with this combination.

Fennel might slow blood clotting. Taking fennel along with medications that also slow blood clotting might increase the risk of bruising and bleeding.

Tamoxifen (Nolvadex)

Interaction Rating=Moderate Be cautious with this combination.

Large amounts of fennel seem to affect estrogen levels in the body. Taking fennel along with tamoxifen might decrease the effects of tamoxifen.

Interactions with herbs & supplements

Herbs and supplements that might slow blood clotting: Fennel might slow blood clotting and increase the risk of bleeding. Taking it with other supplements with similar effects might increase the risk of bleeding in some people. Examples of supplements with this effect include garlic, ginger, ginkgo, nattokinase, and Panax ginseng.

Interactions with foods

There are no known interactions with foods.
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This monograph was last reviewed on 31/07/2023 10:00:00 and last updated on 02/09/2022 03:45:10. Monographs are reviewed and/or updated multiple times per month and at least once per year.
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