Fennel (Foeniculum vulgare) seeds contain the volatile oil composed largely of anethole, which is a phytoestrogen, as well as fenchone, estragole, 1,8-cineole (eucalyptol), and other constituents. Anethole is excreted in breastmilk.[1] Fennel is a purported galactogogue that has been used in traditional medicine as well as being included in some proprietary mixtures promoted to increase milk supply.[2-14] Two small studies found an increase in some parameters such as milk volume, fat content and infant weight gain with fennel galactogogue therapy. However, no increase in serum prolactin has been found with fennel use in nursing mothers.[15] Galactogogues should never replace evaluation and counseling on modifiable factors that affect milk production.[16,17] Immersing the breast in a warm infusion of fennel seeds and marshmallow root has been suggested as a treatment for breast inflammation,[18] but no scientific data are available that support this use. Fennel is generally well tolerated in adults, and fennel oil is "generally recognized as safe" (GRAS) for use in food by the U.S. Food and Drug Administration. It has been safely and effectively used alone and with other herbs in infants for the treatment of colic,[19-21] so the smaller amounts in breastmilk are likely not to be harmful with usual maternal doses. Some sources recommend limiting the duration of treatment to 2 weeks. Excessive maternal use of an herbal tea containing fennel, anise and other herbs appeared to cause toxicity in 2 breastfed newborns that was consistent with toxicity caused by anethole, which is found in fennel and anise. Fennel can cause allergic reactions after oral or topical use affecting the respiratory system or skin, including photosensitivity. Diarrhea and hepatomegaly occurred in a woman taking fennel, fenugreek, and goat's rue as galactagogues.[22] Elevated liver enzymes occurred in another woman taking Mother's Milk Tea, which contains fennel.[23] Avoid excessive sunlight or ultraviolet light exposure while using this herbal. Mothers should avoid fennel if they or their infants are allergic to carrots, celery, or other plants in the Apiaceae family because of possible cross-allergenicity. Dietary supplements do not require extensive pre-marketing approval from the U.S. Food and Drug Administration. Manufacturers are responsible to ensure safety, but do not need to prove the safety and effectiveness of dietary supplements before they are marketed. Dietary supplements may contain multiple ingredients, and differences are often found between labeled and actual ingredients or their amounts. A manufacturer may contract with an independent organization to verify the quality of a product or its ingredients, but that does not certify the safety or effectiveness of a product. Because of the above issues, clinical testing results on one product may not be applicable to other products. More detailed information about dietary supplements is available elsewhere on the LactMed Web site.