Fennel contains anethole, which can explain some of its medical effects: it, or its polymers, act as phytoestrogens.
Mrs. Grieve's Herbal states:
On account of its carminative properties, fennel is chiefly used medicinally with purgatives to allay their side effects, and for this purpose forms one of the ingredients of the well-known compound liquorice powder. Fennel water has properties similar to those of anise and dill water: mixed with sodium bicarbonate and syrup, these waters constitute the domestic 'gripe water', used to correct the flatulence of infants. Volatile oil of fennel has these properties in concentration.
Fennel tea, also employed as a carminative, is made by pouring boiling water on a teaspoonful of bruised fennel seeds.
Fennel can be made into a syrup to treat babies with colic (formerly thought to be due to digestive upset), but long term ingestion of fennel preparations by babies is a known cause of thelarche.
For adults, fennel seeds or tea can relax the intestines and reduce bloating caused by digestive disorders.
In the Indian subcontinent, fennel seeds are also eaten raw, sometimes with some sweetener, as it is said to improve eyesight.Ancient Romans regarded fennel as the herb of sight. Root extracts were often used in tonics to clear cloudy eyes. Extracts of fennel seed have been shown in animal studies to have a potential use in the treatment of glaucoma.
Blood and urine
Some people use fennel as a diuretic, and it may be an effective diuretic and a potential drug for treatment of hypertension.
There are historical anecdotes that fennel is a galactogogue, improving the milk supply of a breastfeeding mother. This use, although not supported by direct evidence, is sometimes justified by the fact that fennel is a source of phytoestrogens, which promote growth of breast tissue. However, normal lactation does not involve growth of breast tissue. There is a single case report of fennel tea ingested by a breastfeeding mother resulting in neurotoxicity for the newborn child.
Syrup prepared from fennel juice was formerly given for chronic coughs. It is one of the plants which is said to be disliked by fleas, and powdered fennel has the effect of driving away fleas from kennels and stables.