Avoid in individuals with a known allergy or hypersensitivity to fennel or other members of the Apiaceae family including carrot, celery, and mugwort because of the chance of cross-sensitization. Oral allergy syndrome has been reported with the use of fennel in a woman. Allergic reactions affecting the skin such as atopic dermatitis and photosensitivity may occur in patients who consume fennel.

Fennel is generally well tolerated. Allergic reactions, such as atopic dermatitis and photosensitivity, are the most common adverse effects but rarely occur. Fennel oil has generally recognized as safe (GRAS) status for food use in the United States. A maximum level of 0.119% is allowed in meat products.

Epileptic seizures have been reported with the use of fennel oil. Respiratory problems including bronchial asthma, hay fever, occupational rhinoconjunctivitis (inflammation of the lining of the nose and the mucous membrane that covers the front of the eye and lines the eyelids) and asthma have been reported in patients working with fennel seed.

Inhalation of essential oils, including fennel oil, resulted in 1.2-2.5-fold increase in relative sympathetic activity, representing low frequency amplitude of systolic blood pressure.

Use cautiously in diabetic patients. Fennel honey syrup is a source of carbohydrates.

Yersinia enterocolitica, a bacterial pathogen, has been isolated in the Umbelliferae family, which could pose a potential threat of infection if fennel is consumed fresh. Fennel preparations, other than fennel seed infusions and fennel honey, should be avoided in infants and toddlers.

The constituent, estragoel, is a procarcinogen (precursor of a cancer causing compound).

Fennel is not recommended in pregnant or breastfeeding women due to a lack of available scientific evidence. Based on expert opinion, fennel preparations, other than fennel seed infusions and fennel honey, are contraindicated during pregnancy.

There is no proven safe or effective dose of fennel. For angiotensin converting enzyme inhibitor-induced cough, 1-1.5 grams of fennel fruit has been used up to three times daily. Up to 4,600 micrograms has been studied for its antioxidant effects. For dysmenorrhea (painful menstruation), 25 drops of a 2% concentration of fennel fruit has been taken every four hours for five days.

Traditionally, numerous other doses and preparations have been used, in the form of tea, seed, tincture, oil or dry extract.

There is no proven safe or effective dose of fennel in children. For infantile colic (ages of 2 to 12 weeks), a 0.1% fennel seed oil in a water emulsion and 0.4% polysorbate-80 has been studied for one week. Traditional dosing for upper respiratory tract catarrh (inflammation of mucous membranes) in children is 0.5 gram of the oil per kilogram. For ages 1 to 4 years, 3-6 grams has been used daily; for ages 4 to 10 years, 6-10 grams has been used daily.

Giant fennel (Ferula communis) may increase the risk of bleeding when taken with drugs that increase the risk of bleeding. Some examples include aspirin, anticoagulants ("blood thinners") such as warfarin (Coumadin®) or heparin, anti-platelet drugs such as clopidogrel (Plavix®), and non-steroidal anti-inflammatory drugs (NSAIDS) such as ibuprofen (Motrin®, Advil®) or naproxen (Naprosyn®, Aleve®).

Concurrent use of fennel and ciprofloxacin (Cipro®) may lead to decreased bioavailability of ciprofloxacin. Theoretically, fennel may also interfere similarly with other fluoroquinolone antibiotics.

Fennel may increase the risk of bleeding when taken with herbs and supplements that are believed to increase the risk of bleeding. Multiple cases of bleeding have been reported with the use of Ginkgo biloba, and fewer cases with garlic and saw palmetto. Numerous other agents may theoretically increase the risk of bleeding, although this has not been proven in most cases.