What Are The Side Effects Of Fennel Seeds?

Save yourself from allergies and other issues caused by overconsumption of this herb.

Reviewed by Yvonne O’ HalloranYvonne O’ Halloran, MND
Fact-checked by Moksha GandhiMoksha Gandhi, BPharm, Certified Health & Wellness Coach
Edited by Ravi Teja TadimallaRavi Teja Tadimalla, Professional Certificate In Food, Nutrition & Health
Written by Sindhu KogantiSindhu Koganti, Certified Health & Nutrition Life Coach  • 

You may have heard that chewing fennel seeds can improve your digestion. Eating fennel (a perennial herb native to the Mediterranean region) seeds in moderation has a multitude of benefits. But what happens when you eat them in excess amounts? Are there any side effects of fennel seeds?

When you eat the recommended dose of fennel seeds, they are quite safe. But their overconsumption can cause phytophotodermatitis, premature thelarche, contact dermatitis, and /or an allergic reaction. Read on to find out the side effects of fennel seeds!

protip_icon Know The Flip Side: Fennel Seeds

Short-Term Effects
Chest and throat tightness, labored breathing, nausea, vomiting, rashes, and irregular menstruation.

Long-Term Effects
Premature breast development and the onset of skin conditions like contact dermatitis and phytophotodermatitis.

Drug Interactions
May interact with anticoagulant, antibiotic, and antiplatelet drugs.

When To See A Doctor
If you see allergic reactions like skin rashes, hives, shortness of breath nausea, and vomiting or experience seizures and disturbed bowel movements.

Fennel Seeds Side Effects

  1. May Cause Phytophotodermatitis
  2. May Interact With Drugs
  3. May Cause Thelarche
  4. May Cause Contact Dermatitis
  5. May Cause Allergy
  6. May Stimulate The Uterus
  7. May Inhibit Drug-Metabolizing Enzymes
  8. Other Toxicities

1. May Cause Phytophotodermatitis

A hand showing photodermatitis
Image: Shutterstock

Fennel seeds contain phototoxic compounds called psoralens. When the skin comes in contact with these compounds, and this is followed by exposure to sunlight, it results in the formation of blister-like patches and inflammation on your skin. This condition is called phytophotodermatitis (1). Other symptoms of phytophotodermatitis include pain and redness, and, in some extreme cases, chills, fever, dizziness, and headache.

protip_icon Quick Tip
One may experience phytophotodermatitis within 24 hours of the excess consumption of fennel seeds, which may then peak in the next 48 hours to 72 hours. Symptoms include blisters that cause black or purple scars(10).

2. May Interact With Drugs

Girl having epileptic seizure
Image: Shutterstock

If you are under medication for seizure disorders or epilepsy,  fennel seeds are a big no. The seeds interact with the functioning of the drug and may aggravate the situation (2).

3. May Cause Premature Thelarche

Thelarche is a condition that marks the start of breast development in women at the beginning of puberty. Fennel can cause premature thelarche.

In a study, consumption of two to three teaspoons of fennel tea every day had led to breast development in a 12-month-old girl (3). The breast development regressed and became normal by the end of one year after stopping fennel consumption.

4. May Cause Contact Dermatitis

Fennel seeds may cause contact dermatitis
Image: IStock

Contact dermatitis is a skin condition, the symptoms of which include skin inflammation and an outburst of blisters. Fennel seed oil is known to cause contact dermatitis and other cross-reactions (4).

5. May Cause Allergy

Woman scratching her allergic neck
Image: Shutterstock

Fennel seeds can cause allergies. Mostly, people with pre-existing allergies are vulnerable to allergies caused by fennel (5). Abdominal cramps and wheezing are the other symptoms of allergic reactions.

protip_icon Quick Tip
It is recommended to avoid carrot, parsley, and anise if you are allergic to fennel seeds. They all belong to the same family of plants known as Apiaceae.

6. May Stimulate The Uterus

One of the most talked-about fennel seeds’ side effects is that they stimulate the uterus and also cause menstrual irregularities (6). Fennel seeds should not be consumed by pregnant women as the seeds can stimulate uterine contractions and cause premature delivery.

7. May Inhibit Drug-Metabolizing Enzymes

A bowl of fennel seeds
Image: IStock

A constituent of fennel seeds is known to inhibit a drug-metabolizing enzyme called cytochrome P450 3A4 (7). This reduces the effectiveness of medications that are metabolized by these enzymes. Hence, not consuming fennel seeds while under such medication is the best option.

8. Other Toxicities

Fennel seed oil was found to be detrimental to DNA. Also, a constituent of fennel called estragole was found to cause tumors in animals (8). If you have any kind of hormone-sensitive disease like cancer, strict medical advice is mandatory before consuming fennel.

Infographic: 4 Side Effects Of Consuming Excess Of Fennel Seeds

Fennel is a unique spice because it also acts as a mouth freshener in addition to improving the flavor and scent of food. It is also popular in Ayurveda. But did you know that excessive fennel consumption can be hazardous to your health? Take a look at the infographic below for more details.

4 side effects of eating excess of fennel seeds (infographic)

Illustration: StyleCraze Design Team

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Spices such as fennel seeds are used for their aromatic properties and to aid digestion after meals. However, overconsumption can lead to a number of problems. The side effects of fennel seeds range from premature breast development to the onset of contact dermatitis. It may trigger phytophotodermatitis or stimulate uterine contractions. In some cases, fennel seeds may trigger allergic reactions or interfere with the functioning of certain medications, especially those for seizures and epilepsy. Avoid such complications by limiting your intake or abstaining from it altogether.

Frequently Asked Questions

Is it ok to eat raw fennel?

Yes, it is ok to eat raw fennel. Fennel has a crunchy texture and a mildly sweet flavor, which make it a pleasant addition to any dish.

Do fennel seeds increase body heat?

No. The seeds have a cooling effect on the body. You can have a fennel seed drink in summer to feel cool and refreshed.

How can you tell if fennel has gone bad?

If fennel turns discolored and soft or develops brown marks on its white layers, it indicates spoilage. Also, if fennel smells off-putting, discard it.

How long does fennel last in the fridge?

It lasts 5 to 10 days if you are getting it directly from the garden. Trim the fronds to two or three inches above the bulb of fennel. Wrap loosely in a plastic bag and store in the fridge.

Are fennel seeds harmful for lactating women?

Though fennel is a purported galactagogue (a food that promotes or increases the flow of a mother’s milk), excess consumption can cause toxicity in newborns and diarrhea and liver damage in the mother (9).

Does fennel affect blood pressure?

There are currently no studies that suggest that fennel has a negative effect on blood pressure. However, research suggests that fennel is commonly used to treat hypertension (high blood pressure) (10).

Does fennel seed affect thyroid?

Research shows that long-term exposure to fennel can cause damage to the thyroid gland. It can lead to a reduction in thyroid hormone serum levels which may cause hypothyroidism (11).

Do fennel seeds increase estrogen?

Research indicates that fennel seeds have very little impact on estradiol levels (a type of estrogen) (12).

Does fennel cause constipation?

No, there are currently no studies that suggest that fennel seeds may cause constipation. Fennel may help improve bowel movement and help relieve constipation (13).

Does fennel cause digestive problems?

No, studies suggest that fennel seeds help relives gas, bloating, heartburn, and even help with nausea and vomiting. It even helps with kidney damages (14).

Key Takeaways

  • Fennel seeds contain the compound psoralen that may exacerbate sun damage, leading to skin irritation and inflammation.
  • If consumed in high amounts, it can interfere with antiepilepsy or seizure-related drugs, rendering them ineffective.
  • They may trigger premature labor in expectant women as certain compounds may signal uterine muscles to contract.
  • Fennel seeds can cause premature thelarche in young girls.

Discover the hidden side effects of fennel. From digestive troubles to hormonal imbalances, this eye-opening video reveals the surprising downsides of this spice.


Articles on StyleCraze are backed by verified information from peer-reviewed and academic research papers, reputed organizations, research institutions, and medical associations to ensure accuracy and relevance. Check out our editorial policy for further details.
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