Fennel might be the cause of an oral allergy syndrome, which is an allergic reaction to plant proteins. Some persons who are allergic to pollen may be affected by this occurrence. Following the eating of the offending meal, itching and burning feelings arise in the mouth, lips, and throat. Symptoms might fade away in a matter of minutes. However, you should see your doctor, who can refer you to an allergist, to evaluate the source of the response and any preventative precautions that should be taken.