Antihistamines, which are commonly used to treat allergy symptoms, interfere with skin tests, as they can prevent the skin from reacting to the allergens being tested. People who take an antihistamine need either to choose a different form of allergy test or to stop taking the antihistamine temporarily before the test. The period of time needed can range from a day or two to 10 days or longer, depending on the specific medication. Some medications not primarily used as antihistamines, including tricyclic antidepressants,