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Q: 
Can medications affect the validity of a skin prick/puncture test? Would being on certain medications affect how strongly a person reacts during a skin test?
A: 
Yes, several medications can affect skin testing responses. An allergist will always do a positive and a negative skin test control, which helps to determine if a medication that would interfere with the allergic response has inadvertently been taken.

Antihistamines are the drugs most commonly associated with suppression of the skin test response. Any medication that suppresses histamine will reduce or prevent the swelling and redness at the skin testing site. Swelling and redness at the skin test site indicate that a patient is allergic to the allergen being tested.

For best results from your skin test, you should avoid most antihistamines for at least 7 days prior to skin testing. Many guidelines only recommend 3-4 days of avoidance prior to skin testing, but a significant number of patients will still have some reduced skin test response for up to 7 days.

Exceptions to the 7-day rule include hydroxyzine (“Atarax”), which should be stopped 10 days prior to allergy testing, and diphenhydramine (“Benadryl”), which can be taken until 48 hours prior to allergy testing.

Patients who are taking psychiatric medications (such as amitriptyline, quetiapine, doxepin, and imipramine) can also have skin test suppression. However, no patient should stop these medications without discussing this decision with their psychiatrist or primary care physician. The allergist can help the patient decide if they need to stop the psychiatric medication, or if an alternative method of allergy testing should be considered.

Another group of medications, called beta-blockers, are often prescribed for high blood pressure. While these medications will not interfere with the skin test response, they can introduce more risks for allergy testing in the case of a serious reaction. Beta-blockers can slow the patient’s response to epinephrine, which is used for treatment in event of an allergic emergency. This is a very rare event, but most allergists prefer for patients to be off beta-blockers for allergy testing. This decision should be made with an individual patient, and only after the allergist discusses the matter with the patient’s cardiologist or primary care physician to determine which course of action holds the least risk for the patient. As with antihistamines, ideally the beta-blockers should be stopped at least 5-7 days prior to allergy testing.