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Fennel - Uses and Side Effects

By:   |   Jul 08, 2018   |   Views: 26   |   Comments: 0

Fennel was known to the ancient Chinese, Indian, Egyptian, and Greek civilizations. It was in great demand during the Middle Ages. The plant was introduced to North America by Spanish priests and the English brought it to their settlements in Virginia. All parts of the plant have been used for flavorings. The oil has been used to protect stored fruits and vegetables against infection by pathogenic fungi. Fennel has been used for treatment of gastroenteritis and indigestion, to stimulate lactation, as an expectorant and emmenagogue and, reportedly, as an antidote to poisonous herbs, mushrooms and snakebites.

Fennel oil is obtained from the ripe or dried seeds of either sweet or bitter fennel. The composition of the oil varies slightly, depending on the source. Fennel oil extracted from bitter fennel is made up primarily of 50% to 75% trans anetholes, 12% to 33% fenchone, and 2% to 5% estragole. Fennel oil extracted from sweet fennel is made up of 80% to 90% trans-anetholes, 1 % to 10% fenchone, and 3% to 10% estragole. Additional components are present in smaller quantities. Fennel oil stimulates GI motility, and at high levels it has antispasmodic activity. The anethole and fenchone components have a secretolytic effect on the respiratory tract, probably a result of fennel's local irritant effects on the respiratory tract. Fennel is available as essential oil, honey syrup, seeds, and herbal tea.

Reported uses

Fennel is used as an expectorant to manage cough and bronchitis. Also used to treat mild, spastic disorders of the GI tract, feelings of fullness, and flatulence. Fennel syrup has been used to treat upperrespiratory tract infections in children.


Essential oil: 0.1 to 0.6 ml by mouth every day, up to 2 weeks

Honey syrup with 0.5 g fennel oil/kg: 10 to 20 g by mouth every day, up to 2 weeks

Seeds (crushed or ground, used for teas or other beverages): 5 to 7 g by mouth every day, up to 2 weeks.


Adverse reactions associated with fennel oil include hallucinations, nausea, and vomiting, photodermatitis, contact dermatitis, and allergic reaction. There is an increased risk of seizures when given with drugs that lower the seizure threshold or anticonvulsants. There is also an increased risk of photosensitivity reaction with fennel use.

Those with sensitivity to fennel, celery, or similar foods and herbs should avoid use. Pregnant patients, small children, and those with a history of seizures should also avoid use. Diabetic patients should use the honey syrup cautiously because of the sugar content.

Safety Risk The use of fennel oil has been associated with seizures and pulmonary edema.

Clinical considerations

Ask patient whether he has an allergic response to celery, fennel, or similar spices and herbs.

If patient has diabetes, warn him about the sugar content of the product.

Tell patient to stop taking this herb and contact health care provider immediately if he experiences difficulty breathing, hives, or a rash.

Tell patient to remind pharmacist of any herbal or dietary supplement that he's taking when obtaining a new prescription.

Advise patient to consult his health care provider before using an herbal preparation because a conventional treatment with proven efficacy may be available.

Safety Risk Advise patient that the maxImum length of use shouldn't exceed 2 weeks. Don't mistake poison hemlock for fennel. Hem-lock can cause vomiting, paralysis, and death. Know the source of preparation before administering fennel.

Research summary

The concepts behind the use of fennel and the claims made regarding its effects haven't yet been validated scientifically.

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