The Pros and Cons of Fennel
Fennel is a spice and a medicinal plant with a long tradition. It decreases inflammation, relieves menstrual pain, may help with stomach/gut disorders, and may even improve memory, mood, and a number of disorders and diseases. However, its regular use should be avoided because it can cause some serious side effects. Read on to learn more about the many 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 a medicine.
It originated in the Mediterranean, but today it is cultivated all over the world [R].
Traditionally, it has been used to treat cough/cold, fever, cuts, stomach aches, nausea, flatulence, diarrhea, constipation, insomnia, arthritis, conjunctivitis, colic in children, and to increase breast milk production [R, R].
In Asian cultures, it was ingested to recover from snake bites, as it was believed that it helped eliminate poisons from the body [R].
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.
- Anethole – this is the main active ingredient of fennel. It is antimicrobial (kills germs), and also mimics estrogen, and increases prolactin.
- Flavonoids like quercetin and apigenin – these are antioxidant, antibacterial, and anti-inflammatory.
- Phenolic compounds such as rosmarinic acid and chlorogenic acids – these are antioxidant and anti-inflammatory.
- Terpenes such as fenchone and limonene, which improve wound healing.
- Water-soluble vitamins like ascorbic acid (vitamin C), thiamine (vitamin B1), riboflavin (vitamin b2), niacin (vitamin B3), and pyridoxine (vitamin B6).
- Fat-soluble vitamins such as vitamins A, E, and K.
- Trace minerals and other elements like aluminum, barium, calcium, cadmium, cobalt, chromium, copper, iron, magnesium, manganese, nickel, lead, strontium, and zinc.
- Essential amino acids like leucine, isoleucine, phenylalanine, and tryptophan.
- Dietary fiber.
Mechanism of Action
Fennel exerts its beneficial effects by:
- Mimicking estrogen. Fennel contains compounds such as anethole that mimic estrogen’s function in the body [R].
- Increasing prolactin (also due to anethole) [R].
- Preventing oxidative stress by increasing the content/activities of liver detoxification enzymes (phase I and phase II), and the activity of antioxidant enzymes superoxide dismutase (SOD) and catalase [R, R].
- Decreasing inflammation by reducing the production of inflammatory cytokines IL-1beta, IL-6, TNF-alpha, and inflammatory agents NF-κB, MMP2, MMP9, and 5-LOX [R, R, R].
- Increasing proteins that cause cancer cell death, such as TIMP1, caspase-3, caspase-9, p21, and p27 [R, R].
- Increasing levels of acetylcholine (a major neurotransmitter) by blocking acetylcholinesterase [R].
- Increasing the production of collagen, elastin, TGF-β1, Nrf2, and GSH in the skin [R].
Health Benefits of Fennel
1) Fennel May Help with 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.
There are few studies that support these benefits. This doesn’t mean that the benefits are not there, just that few have studied them.
Fennel seed oil reduces intestinal spasms and increases gut flow [R].
2) Fennel Helps with Infantile Colic
“Colic” is the term used to refer 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.
Because fennel is known to have a soothing effect on the gastrointestinal system, it has a long history as a folk remedy for infantile colic. This use has also been backed up by science.
For example, 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 [R].
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 [R, R].
3) Fennel Mimics Estrogen
Fennel has estrogen-like activity (it is estrogenic).
Because of this, it can reduce menstrual pains, alleviates symptoms of menopause, and increases libido [R].
Fennel Relieves Menstrual Pain
Fennel capsules shortened the length of the menstruation cycle, reduced period-related nausea, and improved subjective well-being in 40 young women compared to a group receiving placebo treatment [R].
Fennel was also 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 [R].
Fennel May Help with PMS
This herb reduced the severity of symptoms in 90 young women with moderate to severe PMS [R].
Fennel Is Beneficial in Menopause
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 [R].
Fennel May Help Avoid 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 of this are not yet known, fennel has shown potential in reversing this distressing condition.
4) Fennel May Promote Breast Milk Production
It has been traditionally used by breastfeeding mothers to increase breast milk production [R].
While there are no studies that have directly tested the effects of fennel itself on milk production, there is still some scientific evidence to explain why it might have a beneficial effect on breast milk production.
Dopamine blocks prolactin, a hormone that stimulates breast milk production. Anethole, found in fennel, may compete with dopamine at dopamine receptors, thereby blocking the inhibitory action of dopamine on prolactin [R]. This would result in more milk production.
A study of 46 women showed that fennel capsules increase blood prolactin levels [R].
5) Fennel Is Antimicrobial
Historically, fennel has been used as a remedy for many infectious disorders [R].
It may also prevent the growth of Candida albicans, and harmful molds (Aspergillus niger and Fusarium oxysporum) [R].
6) Fennel Has Antioxidant Effects
However, wild fennel has a higher free radical scavenging activity and a higher content of phenolic and flavonoid compounds than medicinal and edible fennel — so anyone interested in using this herb for its antioxidant effects should try to get their hands on the wild fennel variety [R].
7) Fennel Is Good for the Skin
A cream containing fennel extract improved skin texture and increased skin water content in 11 volunteers [R].
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 [R].
8) Fennel May Help Reduce Inflammation
This herb suppressed inflammation in mice with lung injury, by decreasing the production of the inflammatory cytokines IL-6, TNF-alpha, and the inflammatory agents MMP9 and nitric oxide (NO) [R].
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 [R].
Anethole, found in fennel, decreased inflammatory cytokines IL-1beta and TNF-alpha in a rat model of periodontitis (inflammation of the gums) [R].
9) Fennel May Combat Tumors
Anethole, the principal active component of fennel seeds, increased survival time and reduced tumor weight and volume in mice with cancer [R].
10) Fennel May Alleviate Anxiety and Depression
In a DB-RCT of 60 post-menopausal women, fennel showed borderline significant improvement in treating anxiety and depression [R].
Fennel essential oil decreased anxiety in mice [R].
11) Fennel May Improve Memory
It blocked acetylcholinesterase, an enzyme that breaks down acetylcholine, in mice [R]. This increases acetylcholine and improves the creation of memories.
12) Fennel May Help Diabetes
Fennel essential oil reduced blood sugar levels in rats with diabetes by nearly 50%. It also prevented the diabetes-induced damage to kidneys and pancreas [R].
13) Fennel May Decrease Bad and Increase Good Cholesterol
Another study noticed the increase in HDL-cholesterol in rats [R].
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.
14) Fennel May Protect the Liver and Kidneys
15) Fennel May Decrease Blood Pressure
16) Fennel May Decrease Blood Clots
Fennel essential oil prevents blood clotting in guinea pig blood and in mice [R].
17) Fennel May Help with Wound Healing
18) Fennel May Help Soothe Pain
The herb, particularly its ingredient anethole, decreased pain in animals without causing sedation. They most likely do this by decreasing inflammatory mediators, which can worsen the sensation of pain [R, R].
19) Fennel May Help Reduce Appetite
20) Fennel May Increase Airway Relaxation
Fennel extract and essential oil had a significant ability to relax the airways (trachea) of guinea pigs [R].
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 respiratory system to improve airflow to the lungs (bronchodilation) [R]. Anethole, one of the ingredients of fennel, may mimic the effects of dopamine on the respiratory system, thereby achieving a similar effect.
21) Fennel May Help Osteoporosis
However, in a DB-RCT in 60 postmenopausal women, short-term (one month) fennel treatment caused no changes in bone density [R].
Further studies with longer durations are needed to confirm a potential benefit of this herb on bone density in humans.
In most clinical studies, fennel capsules containing 100 mg of active ingredients were given to subjects 2 or 3 times per day.
When given in the form of tea, this herb is usually taken half an hour before a (to improve digestion), and up to 3 times a day for other purposes.
Fennel seeds are also commonly used as a spice in cooking.
Fennel has a great safety profile in clinical trials, with few and minor side effects. It was also safe in studies on infants, although in such cases it is usually only used for relatively short periods of time [R].
However, although the occasional or short-term use of this herb has some indisputable benefits, regular use should be avoided – mainly because anethole in fennel mimics estrogen, which can cause a hormonal imbalance in the body.
Fennel Can Cause Premature Sexual Development
However, after cessation of fennel, the breast development gradually returns to normal [R].
Fennel May Cause Hyperprolactinemia (High Prolactin)
Fennel increases prolactin levels [R].
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.
Fennel 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.
Fennel Can Cause Allergies
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 [R].
Fennel Can Causes 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 [R].
However, this occurs rarely.
Fennel Contains Estragole (Which Can Cause Cancer)
Fennel also contains estragole, a compound that has been shown to cause cancer. Many other herbs and their essential oils also contain this compound [R].
However, research indicates that pure estragole is deactivated by many other substances contained in these plants [R].
Human studies report very few and rare side effects associated with the short-term use of this herb [R].
Among people who took larger doses, some also say they experienced difficulty breathing and an irregular heartbeat.
Bleeding Disorders (or Being On Blood Thinning Medication)
Fennel slows blood clotting [R].
Therefore, it may prolong bleeding and delay wound healing in people with bleeding disorders, or those on blood thinning medication.
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 these possibilities.
Fennel blocks 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 or other unpredictable effects [R, R].
According to users, fennel seeds, capsules, and teas are great for bloating, gas and stomach issues. Some also use it while breastfeeding.
- “Excellent for relaxing the swollen stomach.”
- “A great product for IBS sufferers – reduces bloating.”
- “I suffer from IBD and quite a few food intolerances, and fennel really makes my stomach feel better. These capsules are cheap and seem to work well.”
- “This tea is not only pleasant to drink but has become my go-to tummy rescue remedy. Can not recommend enough!! Absolutely amazing!”
- “It helps milk production for nursing.”
- “I’m breastfeeding my little one and I have been taking these tablets to help him bring the wind up easier after a feed. Absolutely fantastic is all I can say!!”
- “The oil helps heal wounds because of antibacterial properties. Added to a carrier oil and rubbed onto stomach or feet it helps relieve muscle spasms in the gut and is very helpful for digestion.”
However, frequent use may cause some side effects due to its estrogen-like activity. Some may also experience allergies.
- “If you are a man, you should not be taking this on a regular basis. I found out the hard way that daily use of this product can cause undesirable hormonal changes in a male and enlarged prostate.”
- “After finding a fibroid I stopped all fennel.”
- “Caused mouth swelling.”
- Capsules (Amazon or iHerb)
- Organic Tea (Amazon)
- Seeds (Amazon or iHerb)
- Essential Oil (Amazon or iHerb)