Fennel is a perennial herb native to southern Europe and Asia Minor and cultivated in temperate regions around the world. Growing to roughly one meter in height, fennel has alternate, feathery leaves which are divided into several threadlike segments. The plant flowers from July to October, bearing flat-topped clusters of small, yellow flowers, which eventually give way to greenish-brown, oval fruits (commonly called seeds) about 6 millimeters long with prominent ridges. The dried fruits and the oil extracted from them are used for health purposes and as a flavoring agent in foods, confections, liqueurs, and medicines.
The primary active component of fennel is its volatile oil, which consists mostly of a compound known as trans-anethole, but also contains smaller amounts of fenchone, estagole, limonene, camphene, and alpha-pinene.
While fennel remains a highly popular spice, it is also used medicinally as a digestive aid and antiflatulent. Germany's Commission E lists fennel as an approved herb for treatment of mild gastrointestinal afflictions, flatulence, and respiratory tract health.
The daily dosage recommended by Commission E is five to seven grams of dried fennel fruits (seeds), 0.1to 0.6 milliliters of fennel oil, or equivalent preparations. Fennel appears to be very safe at this dosage range, although some individuals may experience allergic reactions of the skin and respiratory tract. Because fennel oil has been associated with more serious side effects, some herbalists recommend use of the fruit over the isolated volatile oil.