IgE-mediated reactions
Fennel seed may induce symptoms of food allergy, rhinoconjunctivitis and asthma in sensitised individuals. (1)
Occupational allergic rhinoconjunctivitis and asthma to Fennel seed have been reported. (1)
Typical features of oral allergy syndrome (OAS) may occur after the ingestion of Fennel. (8) Among 196 Birch pollen-hypersensitive patients with OAS caused by different vegetables who were examined in a cross-sectional part of a study, 195 had Apple and/or Hazelnut allergy, and 103 had Apiaceae sensitivity; only 1 patient had Apiaceae allergy alone. Apiaceae-positive patients showed significantly higher Birch pollen-specific IgE levels than did negative ones. (7) This may be applicable to Fennel seed as well.
Skin-specific IgE tests to native spices have been carried out in patients suspected of food allergies to spices. Frequent sensitisation to Apiaceae (Coriander, Caraway, Fennel, and Celery) was observed, with 32% of children and 23% of adults demonstrating the presence of specific IgE to these spices. (9, 10 )This may be applicable to Fennel seed as well.
Other reactions
A survey of the literature shows essential oils of 11 plants to be powerful convulsants (Eucalyptus, Fennel, hyssop, pennyroyal, rosemary, Sage, savin, tansy, thuja, turpentine, and wormwood) due to their highly reactive monoterpene ketones. (11)
The oils of Fennel seed contain anethole, fenchone, camphene, sabenine, myrcene, limonene, camphor, and several other volatile constituents. Skin contact with the sap or essential oil is said to cause photosensitivity and/or dermatitis in some people. Ingestion of the oil can cause vomiting, seizures and pulmonary oedema.
See also Fennel, fresh f276 (another description of Foeniculum vulgare).