Allergy testing helps identify triggers for allergic reactions. To ensure accurate results, temporarily stopping certain medications, especially antihistamines, is often necessary as they can interfere with the body’s natural response.
The Required Antihistamine-Free Period
The duration for stopping antihistamines before allergy testing typically ranges from a few days to over a week. Most oral antihistamines require cessation 3 to 7 days prior. Longer-acting types like cetirizine (Zyrtec), levocetirizine (Xyzal), loratadine (Claritin), and fexofenadine (Allegra) often need 5 to 7 days, while shorter-acting ones like diphenhydramine (Benadryl) may only need about 48 hours.
Individual medical advice varies based on medication, test type, and health status. Always consult your healthcare provider for personalized instructions before any allergy testing.
Why Antihistamines Interfere with Allergy Tests
Antihistamines counteract histamine, a chemical released by the immune system during an allergic reaction. When allergens enter the body, immune cells release histamine, triggering symptoms like itching, redness, swelling, and sneezing. Antihistamines block histamine from binding to H1 receptors, suppressing the body’s natural inflammatory response.
During skin allergy tests, small amounts of allergens are introduced to the skin to observe a localized reaction. If antihistamines are present, they prevent the typical “wheal and flare” response—a raised, red bump surrounded by a red flush—that indicates sensitivity. This suppression can lead to false-negative results, masking an allergy, as the medication prevents the body’s natural histamine response from being fully active.
Common Allergy Tests That Need Preparation
Preparation involving antihistamine cessation is primarily necessary for skin-based allergy tests. The most common is the skin prick test, also known as a scratch or puncture test.
During this procedure, a small amount of allergen extract is applied to the skin, which is then lightly pricked or scratched to allow the allergen to enter just beneath the top layer. Intradermal tests also require this preparation, involving injecting a small amount of allergen just under the skin.
Both skin prick and intradermal tests rely on the body’s immediate, histamine-mediated immune response. In contrast, blood tests for allergies, such as specific IgE antibody tests, measure allergen-specific antibodies in the bloodstream. These blood tests are generally not affected by antihistamine use, making them suitable if a patient cannot stop medications or has certain skin conditions.
Other Medications and Factors to Avoid
Beyond antihistamines, other medications and factors can influence allergy test results. Certain antidepressants, particularly tricyclic antidepressants (TCAs) like amitriptyline or doxepin, can interfere due to their antihistamine-like properties. Discontinuation for one to two weeks before testing should only be done under strict physician guidance.
Some heartburn medications, specifically H2-receptor blockers like cimetidine (Tagamet) or famotidine (Pepcid), can also have antihistaminic effects and may need to be paused for 1 to 7 days. Over-the-counter cold remedies, motion sickness medications, and some sleep aids often contain antihistamines and should be avoided. Non-medication factors like recent illness, skin conditions (e.g., severe eczema, dermatographism), and certain cosmetic products can also affect test accuracy.
Tips for Managing Symptoms Before Testing
Temporarily stopping antihistamines can lead to a resurgence of uncomfortable allergy symptoms. To manage these without medication, several strategies can help:
Minimizing exposure to known allergens (e.g., staying indoors during high pollen counts, keeping windows closed, using air conditioning).
Showering in the evening to wash away pollen and other allergens.
Using nasal saline rinses or sprays to clear allergens and mucus.
Applying cold compresses to itchy eyes and warm compresses to alleviate sinus pressure.
Drinking warm tea for a sore throat caused by nasal drainage.
If symptoms become severe or unmanageable, contact your healthcare provider for alternative solutions or to adjust the testing schedule.