Fennel is likely safe when taken by mouth in the amounts commonly found in food. For the most part, there is not enough evidence to know whether it is safe for adults when used in medicinal amounts.
Some people can have allergic skin reactions to fennel. People who are allergic to plants such as celery, carrot and mugwort are more likely to also be allergic to fennel. Fennel can also make skin extra sensitive to sunlight and make it easier to get a sunburn. Wear sunblock if you are light-skinned.
Special Precautions & Warnings:
Pregnancy and breast-feeding: Not enough is known about the safety of using fennel during pregnancy. It is best to avoid use.
Children: For the most part, there is not enough evidence to know whether it is safe for children when used in medicinal amounts. However, researchers have studied a combination product (ColiMil) for colic that contains fennel, lemon balm, and German chamomile. This product seems to be safe in infants when used for up to one week.
Allergy to celery, carrot or mugwort: Fennel might cause an allergic reaction in people who are sensitive to these plants.
Bleeding disorders: Fennel might slow blood clotting. Taking fennel might increase the risk of bleeding or bruising in people with bleeding disorders.
Hormone-sensitive condition such as breast cancer, uterine cancer, ovarian cancer, endometriosis, or uterine fibroids: Fennel might act like estrogen. If you have any condition that might be made worse by exposure to estrogen, do not use fennel.