The herb fennel has a long history of use as both food and medicine. Traditionally, it is said to act as a carminative, a term that means that it helps the body expel gas. Other traditional uses include increasing breast milk production, easing childbirth, soothing cough, promoting menstrual flow, soothing indigestion, and enhancing libido. Fennel is also a common ingredient in “gripe water,” a traditional (and highly alcoholic) preparation used for treating infant colic.

Animal and test-tube studieshint at a number of potential medicinal effects of fennel or its constituents, such as relaxing smooth muscles, stimulating the flow of bile, and reducing pain.1,2 However, only double-blind, placebo-controlled studies in humans can actually show a treatment effective. Only one such study of this type has been performed for fennel.

This trial enrolled 125 infants with colic, who received either placebo or fennel seed oil at a dose of 12 mg daily per kg of body weight.3 The results were promising. About 40% of the infants receiving fennel showed relief of colic symptoms, as compared to only 14% in the placebo group, a significant difference. Another way to view at the results involves looking at hours of inconsolable crying. In the treated group, infants cried about 9 hours per week, compared to 12 hours in the placebo group. While these are promising results, confirmation by an independent research group is necessary before the treatment can be accepted as effective.

Previously, a small double-blind, placebo-controlled study found similar benefits with a tea containing fennel as well as other herbs ( chamomile, vervain, licorice, and lemon balm).4

No other proposed uses of fennel have undergone study in double-blind trials. One study commonly cited as evidence that fennel is helpful for menstrual painactually proved nothing at all.5 This was an open trial that compared fennel to the drug mefenamic acid. Because participants and researchers were aware of which treatment was which, the power of suggestion had free play, making the results almost meaningless. (For more information, see Why Does This Database Rely on Double-blind Studies?) Another study of fennel also failed to use a placebo.9.

At one time it was thought that fennel had estrogen-like effects, making it a phytoestrogen.6However, subsequent research has tended to indicate that fennel does not have significant phytoestrogen activity.7