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9 Health Benefits of Fennel + Risks & Side Effects

Written by Biljana Novkovic, PhD | Last updated:
Jonathan Ritter
Puya Yazdi
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology), Puya Yazdi, MD | Written by Biljana Novkovic, PhD | Last updated:

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Fennel

Fennel is a spice and a medicinal plant with a long tradition. It may decrease inflammation, relieve menstrual pain, may help with stomach/gut disorders, and may even improve mood, and a number of other disorders and diseases. However, its regular use should be avoided because it can cause some serious side effects. Read on to learn more about health benefits and important safety concerns associated with this plant.

What is Fennel?

Fennel (Foeniculum vulgare) is a plant in the carrot, celery, and parsley family. It is an aromatic herb that has been widely used as a spice and traditional medicine.

It originated in the Mediterranean, but today it is cultivated all over the world [1].

Traditionally, it has been used for cough/cold, fever, cuts, stomach aches, nausea, flatulence, diarrhea, constipation, insomnia, arthritis, conjunctivitis, colic in children, and to increase breast milk production [1, 2].

In Asian cultures, it was ingested to recover from snake bites, as it was believed that it helped eliminate poisons from the body [2].

Fun facts about fennel:

  • The Greek word “marathon” (for the athletic discipline and the battle the discipline was named after) actually means fennel.
  • Fennel is one of the ancient Saxon peoples’ 9 sacred herbs, credited with powerful healing properties.

Fennel contains many active ingredients, such as [1, 3]:

Mechanism of Action

Based on animal and cell studies, fennel:

Snapshot

Proponents

  • Possibly effective for colic
  • May improve immune stomach/gut discomfort
  • May help with menstrual pain, PMS, and menopause symptoms
  • May help with unwanted hair growth
  • Generally safe with minimal side effects reported
  • A long history of use
  • May improve mood
  • Might reduce appetite
  • May Improve skin health

Skeptics

  • Lack of larger, well-designed clinical trials
  • May cause side effects, including hormonal imbalances, allergies, and seizures
  • May interact with medication

Health Benefits

Possibly Effective For:

1) Colic

“Colic” is a term that refers to when young children cry or are in a state of distress for several hours a day. The causes are unknown, but it is widely believed that stomach cramps and other digestive issues play a key role.

A meta-analysis of 17 studies and a review of 14 clinical trials of supplements for gut disorders concluded that fennel (either as an oil, a tea, or an herbal compound) was effective in treating babies with infantile colic [13, 14].

In a study with 125 infants, fennel seed oil eliminated colic in 65% of infants, compared to an improvement of only 24% in the placebo group [15].

Insufficient Evidence For:

2) Improving Stomach/Gut Discomfort

Fennel has a long history of being used to treat a variety of gut and digestive problems, including stomach aches, flatulence, diarrhea, and constipation.

However, at this point in time, there is insufficient evidence to support these benefits.

In a study with 121 people who had Irritable Bowel Syndrome (IBS), a mixture of fennel and curcumin essential oils decreased abdominal pain and improved quality of life [16].

Anethole, a major component of essential fennel oil, improved stomach function and emptying in rats [17].

Fennel extract prevented stomach ulcers caused by alcohol in rats [18].

A small trial that used fennel in a mixture with curcumin and a couple of animal studies cannot be considered sufficient evidence that fennel improves gut discomfort. Larger, well-designed clinical trials are needed to shed light on any potential benefits when it comes to gut health.

3) Alleviating Menstrual Pain

Fennel was found to alleviate menstrual pain in 30 women reporting particularly painful periods (dysmenorrhea). This effect kicked in between 30 – 120 minutes after taking it. However, 5 women withdrew from this study due to finding its odor unpleasant, and one woman reported a mild increase in the amount of menstrual flow [19].

A combination of fennel extract/vitamin E was more effective than ibuprofen in decreasing the intensity of menstrual pain in a study with 68 female students [20].

In a study of 80 young women, fennel capsules reduced period-related nausea and improved subjective well-being compared to placebo [21].

Studies are promising but few and of low quality. Larger trials are needed to establish if fennel is beneficial when it comes to alleviating menstrual pain.

4) Reducing PMS

There is a single study in which this herb reduced the severity of symptoms in 90 young women with moderate to severe PMS [22].

Other studies are needed to corroborate this effect.

5) Reducing Menopause Symptoms

Menopause is often accompanied by symptoms such as hot flashes, sweating, heart discomfort, sleep problems, depression, irritability, fatigue, anxiety, sexual problems, and joint and muscle discomfort.

In a triple-blind, placebo-controlled trial of 90 women, fennel effectively reduced menopausal symptoms without serious side effects [23].

However, more studies are needed to confirm fennel is effective in alleviating menopausal symptoms.

6) Unwanted Hair Growth in Women

Sometimes, women with regular menstrual cycles and normal levels of male hormones can experience unwanted hair growth, a condition known as hirsutism (for example, inappropriate facial hair growth).

While the underlying causes are not yet known, fennel has shown potential in reversing this distressing condition.

For example, fennel gels and creams applied directly to the affected areas of the skin reduced hair growth and hair thickness in a DB-RCT of 44 women and in another study of 38 women [24, 25].

However, larger, well-designed trials are needed to establish that fennel is beneficial for this condition.

7) Anxiety and Depression

In a DB-RCT of 60 post-menopausal women, fennel showed borderline significant improvement in treating anxiety and depression [26].

Fennel essential oil decreased anxiety in mice [27].

Further human studies with a larger sample size are required to confirm whether fennel has any actual effect on these conditions.

8) Reducing Appetite

In 9 healthy overweight women, fennel tea decreased hunger and increased the feeling of fullness [28].

However, this study is small and insufficient to draw any conclusions. Future studies will hopefully clarify if fennel indeed has an effect on appetite.

9) Improving Skin Health

A cream containing fennel extract improved skin texture and increased skin water content in 11 volunteers [29].

Fennel extract may prevent the visible effects of skin aging that come from sun exposure. For example, it increased the production of collagen, elastin, and TGF-beta1 levels in mice exposed to UVB radiation. Furthermore, it dose-dependently decreased the production of reactive oxygen species (ROS) by increasing Nrf2 and antioxidants such as GSH [12].

Cell studies have also found that fennel and trans-anethole also lighten the skin and may prevent skin blemishes by blocking UV-induced melanin production (melanogenesis) [30].

Additional, preferably larger human studies are needed to explore the potential benefits of fennel on skin health.

Lacking Evidence:

No clinical evidence supports the use of fennel for any of the conditions listed in this section. Below is a summary of the existing human, animal, and cell-based research, which should guide further investigational efforts. However, the studies should not be interpreted as supportive of any health benefit.

Breast Milk Production

Fennel has been traditionally used by breastfeeding mothers to increase breast milk production [1].

However, there are no studies that directly tested the effects of fennel on milk production.

In theory, dopamine blocks prolactin, a hormone that stimulates breast milk production, and anethole, found in fennel, may compete with dopamine at dopamine receptors, thereby blocking the inhibitory action of dopamine on prolactin [1]. This could potentially result in more milk production.

A study of 46 women showed that fennel capsules increased blood prolactin levels [31].

Antimicrobial

Historically, fennel has been used as a remedy for many infectious disorders [1].

Fennel extract prevents the growth of several harmful bacteria in the laboratory (such as staph infection and tuberculosis-causing bacteria, E. coli, and Salmonella) [1].

It may also prevent the growth of Candida albicans, and harmful molds (Aspergillus niger and Fusarium oxysporum) [1].

Antioxidant

This herb is an abundant source of antioxidants such as caffeoylquinic acid, quercetin, and other phenolic and flavonoid compounds [32, 33].

Three weeks of fennel supplementation increased the activity of important antioxidant enzymes superoxide dismutase (SOD) and catalase in rats [5].

Inflammation

Fennel extract has shown significant anti-inflammatory effects in rats and mice. It also prevented slow allergic reactions (type IV allergic hypersensitivity) [5].

This herb suppressed inflammation in mice with lung injury, by decreasing the production of inflammatory cytokines IL-6 and TNF-alpha and the inflammatory agents MMP9 and nitric oxide (NO) [6].

Fennel contains several major ingredients that are inhibitors of 5-LOX, an enzyme that causes inflammatory and allergic responses in the body by producing leukotrienes. Fennel’s ability to inhibit these enzymes may make it useful for preventing inflammatory and allergic reactions [8].

Anethole, found in fennel, decreased inflammatory cytokines IL-1beta and TNF-alpha in a rat model of periodontitis (inflammation of the gums) [7].

Tumors/Cancer

Fennel seeds reduced the growth of skin and stomach tumors in mice. They increased the content/activities of phase I and phase II detox enzymes and the activities of antioxidant enzymes [4].

Anethole, the principal active component of fennel seeds, increased survival time and reduced tumor weight and volume in mice with cancer [34].

Memory

Fennel extract prevented the deficits in memory in rats and mice caused by aging and scopolamine (a drug that causes amnesia by blocking acetylcholine) [35, 11].

It blocked acetylcholinesterase, an enzyme that breaks down acetylcholine, in mice [11]. This increases acetylcholine and improves the creation of memories.

Diabetes

Fennel essential oil reduced blood sugar levels in rats with diabetes by nearly 50%. It also prevented diabetes-induced damage to kidneys and pancreas [36].

In another study, prolonged treatment with fennel distillate improved blood glucose, cholesterol, and glycated hemoglobin in diabetic rats [37].

Cholesterol

In mice with high cholesterol, fennel extract decreased total cholesterol, triglycerides, and LDL-cholesterol (bad cholesterol), while it increased HDL-cholesterol (good cholesterol) [1].

Another study noticed the increase in HDL-cholesterol in rats [5].

However, these effects have only been reported in rodents so far, so it’s an open question whether this herb will have as dramatic an effect on cholesterol in human users.

Liver and Kidney Health

Fennel essential oil protected against liver injury and drug-induced liver and kidney damage in rats [38, 39].

Fennel decreased levels of liver-damage markers aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP) and bilirubin in both studies [38].

Blood Pressure

Fennel extract dose-dependently reduced blood pressure in rats [40, 41].

Blood Clots

Fennel essential oil prevents blood clotting in guinea pig blood and in mice [42].

Wound Healing

Two key ingredients of fennel, fenchone, and limonene improved wound healing in rats. They act as anti-inflammatory and antimicrobial agents and also increase collagen production [3].

Pain

Fennel, particularly its ingredient anethole, decreased pain in animals without causing sedation. It may work by decreasing inflammatory mediators, which can worsen the sensation of pain [43, 5].

Airway Relaxation

Fennel extract and essential oil showed an ability to relax the airways (trachea) of guinea pigs [44].

Dopamine is well known for its role as a neurotransmitter in the brain, but it also plays other roles in the body, such as relaxing the airways to improve airflow to the lungs (bronchodilation) [45]. Anethole, one of the ingredients of fennel, may mimic the effects of dopamine on the respiratory system, thereby achieving a similar effect.

Osteoporosis

Fennel seed prevented bone tissue from being broken down, leading to improved bone density and bone mineral content in mice [46].

However, in a DB-RCT in 60 postmenopausal women, short-term (one month) fennel treatment caused no changes in bone density [47].

Further studies with longer durations are needed to examine the potential benefit of this herb on bone density in humans.

Dosage & Safety

Dosage

Because fennel is not approved by the FDA for any condition, there is no official dose. Users and supplement manufacturers have established unofficial doses based on trial and error. Discuss with your doctor if fennel may be useful as a complementary approach in your case and which dose you should take.

In most clinical studies, fennel capsules containing 100 mg of active ingredients were given 2 or 3 times per day.

When given in the form of tea, this herb is usually taken half an hour before a meal (to improve digestion), and up to 3 times a day for other purposes.

Fennel seeds are also commonly used as a spice when cooking.

Safety

Studies with fennel suggest it is generally safe, with few and minor side effects. It was also safe in infants, although in such cases it was usually only used for relatively short periods of time [15].

However, although the occasional or short-term use of this herb may have some benefits, regular use should be avoided – mainly because anethole in fennel mimics estrogen, which can cause a hormonal imbalance in the body.

Can Cause Premature Sexual Development

Fennel has sometimes been noted to cause premature thelarche, the early development of breasts, in babies and young children who have not yet begun going through puberty [48, 49].

However, after cessation of fennel, breast development gradually returns to normal [49].

May Cause Hyperprolactinemia (High Prolactin)

Fennel increases prolactin levels [31].

This may be good for some people (such as those looking to stimulate breast milk production), but bad for others who might already be struggling with high prolactin levels.

There are no studies or clinical reports where fennel was directly linked to high prolactin, but the potential for unwanted side effects is still there.

May Cause Galactorrhea

Galactorrhea is the name for the spontaneous or inappropriate flow of milk from the breast, unassociated with childbirth or nursing. It can occur due to high prolactin levels.

There aren’t any studies or reports linking fennel to galactorrhea, but the concern often arises on forums online.

Can Cause Allergies

People who are allergic to plants from the same family (celery, carrot, mugwort) may also be allergic to fennel [50, 51].

Fennel can cause occupational allergic rhinoconjunctivitis and asthma in people with unusually frequent exposure to this herb, such as cooks and other workers who handle spice on a highly regular basis [52].

Can Cause Photodermatitis

It contains psoralens, a compound found in many plants and vegetables which can make the skin unusually sensitive to light. This means that large amounts of fennel could make your skin more sensitive to the sun (UV rays), leading to rashes and redness in the skin [53].

Contains Estragole

Fennel also contains estragole, a compound that has been shown to cause cancer. However, many other herbs and their essential oils also contain this compound [54].

Research indicates that pure estragole is deactivated by many other substances contained in these plants [2].

Studies also suggest that estragole may be of little concern when fennel is ingested occasionally and not regularly [55, 56].

Side-Effects & Precautions

Side Effects

Human studies report few and rare side effects associated with the short-term use of this herb [15].

  • Nausea (due to the odor) [57]
  • Allergies [23]
  • Mild increase in menstrual flow [19]

Contraindications

Bleeding Disorders (or Being On Blood Thinning Medication)

Fennel may slow blood clotting [42].

Therefore, it may prolong bleeding and delay wound healing in people with bleeding disorders, or those on blood-thinning medication.

Epilepsy

There are a couple of cases where fennel induced seizures in epileptic patients [58, 59].

Estrogen-Sensitive Disorders and Diseases

Fennel mimics estrogen. If you have any condition that may worsen by exposure to estrogen, do not use it.

Examples of these include breast, uterine or ovarian cancer, endometriosis, and uterine fibroids.

Pregnancy and Breastfeeding

Although fennel is traditionally used in some parts of the world during pregnancy and breastfeeding, there are others who consider it potentially unsafe due to its effects on hormone levels (estrogen and prolactin).

Some people have also claimed that this herb can cause miscarriage, menstrual bleeding, and nerve damage in infants. However, these claims have no scientific backing that we are aware of.

Nonetheless, just as with any other supplement, it’s better to exercise caution and be aware of all possibilities. Always consult your doctor before using any supplements, carefully follow your doctor’s recommendations, and never use fennel instead of what they prescribe.

Drug Interactions

Fennel may block the enzymes CYP2D6 and CYP3A4, which are responsible for breaking down (metabolizing) a number of drugs including opioids, antidepressants, antipsychotics, beta-blockers, antibiotics, and statins. Therefore, taking this herb may enhance the effects of these drugs, potentially causing undesired side effects [60, 61].

User Reviews

The opinions expressed in this section are solely those of fennel users, who may or may not have medical or scientific training. Their reviews do not represent the opinions of SelfHacked. SelfHacked does not endorse any specific product, service, or treatment.

Do not consider user experiences as medical advice. Never delay or disregard seeking professional medical advice from your doctor or other qualified healthcare providers because of something you have read on SelfHacked. We understand that reading individual, real-life experiences can be a helpful resource, but it is never a substitute for professional medical advice, diagnosis, or treatment from a qualified healthcare provider.

According to users, fennel seeds, capsules, and teas are great for bloating, gas and stomach issues such as IBS. Some have also used it while breastfeeding.

However, frequent use may cause some side effects due to its estrogen-like activity. Some may also experience allergies.

About the Author

Biljana Novkovic

Biljana Novkovic

PhD
Biljana received her PhD from Hokkaido University.
Before joining SelfHacked, she was a research scientist with extensive field and laboratory experience. She spent 4 years reviewing the scientific literature on supplements, lab tests and other areas of health sciences. She is passionate about releasing the most accurate science and health information available on topics, and she's meticulous when writing and reviewing articles to make sure the science is sound. She believes that SelfHacked has the best science that is also layperson-friendly on the web.

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