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’ Halloran, MND
By Sindhu 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!

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


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, and headache.

2. May Interact With Drugs

Girl having epileptic seizure


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


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


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.

6. May Stimulate The Uterus

One of the most talked-about fennel seeds’ side effects is that they stimulate the uterus (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


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.

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 issues in the mother (9).


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Sindhu Koganti is a certified health and nutrition life coach and has over 5 years of experience in writing on... more

Yvonne O’ Halloran

(MND (Nutrition & Dietetics))
Yvonne is a passionate dietitian who loves to help people reclaim their health and align their morals and beliefs with... more