Fennel has a long history of use for a variety of purposes, including the treatment of gas and other issues of digestive distress and infant colic, promoting menstruation and lactation, and increasing sexual drive. The University of Pittsburgh Medical Center notes that fennel appears effective for treating colic but that other purported uses lack strong scientific study. Some dosing guidelines exist based on study and traditional use.
Drugs.com notes the typical daily doses are 5 g to 7 g of the seed or 0.1 mL to 0.6 mL of the oil. If using fennel seed, you can take this dose in capsules or use this amount to make tea. The University of Michigan Health System notes a suggested dose of 2 g to 3 g of seeds taken three times a day to address indigestion, heartburn and low stomach acidity. Fennel also comes in tincture form; the University of Michigan Health System notes a daily dose of 5 mL to 10 mL three times a day. Talk to your doctor about an appropriate dosage for infant colic.
Use in Certain Individuals
Do not use fennel if you are pregnant or lactating. It has demonstrated adverse effects, including emmenagogue effects, meaning it could induce a miscarriage. The University of Pittsburgh Medical Center reports at one time fennel was believed to possess estrogenic activity, but more recent research suggests it does not possess "significant" activity. Drugs.com notes a particular component of fennel oil was shown to stimulate tumor growth in animals. If you have any hormone-sensitive diseases or cancer, talk to your doctor about the potential risk of consuming fennel.
If you have liver or kidney disease, always clear the use of any supplement with your doctor, as these organs are particularly vulnerable to what you put in your body.
Its long history of use as a spice suggests fennel is a generally safe substance to consume. Some reported side effects include rash from sunlight exposure and dermatitis. Reports exist of fennel oil inducing hallucinations and seizures.
Fennel does not have any official documented medication interactions. Animal research suggests it might reduce the absorption of the antibiotic drugs ciprofloxacin and fluoroquinolone.