Fennel seeds are used in cooking to add flavor to foods, including sausages and fish dishes. Some people also chew the seeds after meals to help with digestion and limit gas. While the small amounts typically used in food are considered safe, using medicinal amounts of fennel seeds isn't safe for everyone. Check with your doctor before consuming fennel as a supplement.
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Taking fennel may cause side effects, including making you more sensitive to sunlight and giving you a rash. It can also bring on menstruation, so pregnant women should avoid taking fennel. There is some concern that fennel use could cause girls to develop breasts prematurely.
Although it isn't common, some people are allergic to fennel. Using any amount of the seed could bring on an allergic reaction, including trouble breathing, hives, rash, swelling of the lips or face and closing of the throat.
Fennel may interact with certain medications, including certain antibiotics and seizure medications. Taking fennel seeds two hours before or after your medication may help make the interaction between antibiotics and fennel less likely, but it may not entirely eliminate the risk, according to the New York University Langone Medical Center, which recommends avoiding fennel when taking antibiotics in the ciprofloxacin family.
Other Potential Adverse Effects
Fennel seeds contain a substance called estragole, which has been shown to cause liver tumors in mice, according to the European Commission Scientific Committee on Food. The exact effect of estragole from fennel on people still isn't clear. An article published in "Evidence-Based Complementary and Alternative Medicine" in 2012 noted that estragole is metabolized differently in mice and people and that the other components of fennel seeds may counterbalance the effects of the estragole and limit the risks. Further research is needed to verify any potential adverse effects, as the studies in mice used purified estragole and not fennel.