Like many other seeds and spices, fennel seeds can indeed go rancid if not stored properly. Exposure to heat, light, and moisture can degrade the seeds over time, leading to a loss of flavor and nutritional value. However, when stored in a cool, dark, and dry place, fennel seeds can maintain their quality for up to six months. For longer storage periods, refrigeration or freezing in an airtight container can further preserve the seeds’ freshness and potency.

Despite their numerous health benefits, fennel seed side effects need to be recognized. Consumption of fennel seeds may lead to allergic reactions16 in some individuals. These reactions can range from mild to severe, including symptoms such as skin rashes, itching, swelling of the mouth or throat, difficulty breathing, and digestive issues.

In rare cases, a severe allergic reaction known as anaphylaxis may occur, which is a medical emergency. It’s important to note that if you have a known allergy to plants in the carrot family, which includes fennel, you may be more likely to be allergic to fennel seeds. Always consult with a healthcare professional before including fennel seeds in your diet if you have food allergies or sensitivities.

Pregnant people are advised to avoid consumption of fennel seeds, as ingestion may stimulate uterine contractions, potentially increasing the risk of premature labor. Those with bleeding disorders or who are on blood-thinning medications should also be wary, as fennel may slow blood clotting17.

Since the effects of eating fennel seeds are not well-studied in the pediatric population, children should avoid consuming large quantities of this seed unless under the guidance of a health care provider.