People in the Mediterranean have used fennel seeds after meals for generations. The seeds and seed oils help relax the muscles of the digestive tract, which can bring comfort when you feel bloated or full.
The most common ways to take it are chewing whole seeds, steeping a tea, or using a standardized extract in capsules or liquids. Anethole, a compound in fennel oil, gets most of the credit for the muscle-relaxing effect.
Solid human research on fennel is scarce. Culinary amounts pose little risk, but you need to treat concentrated extracts and essential oils with care. Large doses of fennel oil can turn toxic. The plant also carries weak estrogen-like activity, so anyone with an estrogen-sensitive condition should steer clear. Allergic reactions do happen in people who are sensitive to celery, carrot, mugwort, or other members of the Apiaceae family.