Allergy testing, particularly the common skin prick method, determines a patient’s immune system response to specific allergens like pollen, mold, or pet dander. During this test, tiny amounts of potential allergens are introduced beneath the skin’s surface to provoke a localized reaction, resulting in a small, raised bump (a wheal) surrounded by redness. Certain medications suppress this necessary immune reaction by blocking the action of histamine, leading to inaccurate “false negative” results. Consequently, following pre-test instructions to temporarily discontinue specific drugs is paramount to ensure the accuracy of the test results.
Primary Medications to Discontinue
The most common class of medications that must be stopped before skin prick testing are H1 antihistamines. These drugs directly interfere with the body’s ability to produce the visible wheal-and-flare reaction by blocking the histamine receptors on cells. If these medications are active in the system, they prevent the skin from reacting to the test allergens, even if an allergy is present.
This category includes both first-generation and second-generation formulas. First-generation antihistamines, such as diphenhydramine (Benadryl) and hydroxyzine (Atarax, Vistaril), are generally short-acting but have a significant suppressive effect. Second-generation agents, including fexofenadine (Allegra), cetirizine (Zyrtec), loratadine (Claritin), and levocetirizine (Xyzal), are long-acting and are also potent suppressors. The standard withdrawal period for these primary antihistamines typically ranges from five to seven days to ensure they are fully cleared from the patient’s system.
Non-Antihistamine Drugs That Interfere
Beyond primary allergy medications, several other drug classes possess antihistamine-like properties that can mask skin test results, requiring temporary discontinuation. Tricyclic Antidepressants (TCAs), such as amitriptyline and doxepin, are potent inhibitors of the histamine response and should be stopped well in advance. Due to their complex pharmacology and long half-lives, TCAs often require a longer withdrawal period, sometimes up to two weeks. Patients must never stop these medications without guidance from their prescribing physician.
Another unexpected group is the H2 receptor blockers, acid reducers like famotidine (Pepcid) and cimetidine (Tagamet), which can suppress skin reactions. Additionally, many over-the-counter sleep aids or motion sickness remedies contain first-generation antihistamines, such as diphenhydramine or meclizine, and must also be avoided. While oral steroids do not usually interfere with skin test results, topical steroids applied directly to the test area should be stopped for at least a few days to prevent localized suppression.
Preparation Timelines and Alternative Testing
To ensure the reliability of the skin prick test, a general preparation timeline requires stopping all interfering medications for a minimum of five to seven days before the appointment. This timeline accounts for the time it takes for most oral and topical antihistamine agents to clear the body sufficiently to allow a true allergic reaction to occur. Patients should be aware that many combination cold and flu products, as well as certain medicated eye drops and nasal sprays, also contain antihistamines and must be discontinued.
Most non-allergy medications are generally safe to continue, including acetaminophen (Tylenol), non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, and most inhaled asthma medications. For patients who cannot safely interrupt their medication regimen, such as those taking certain psychiatric drugs or who experience severe allergy symptoms when untreated, an alternative is the specific IgE blood test, sometimes called RAST testing. Since blood testing measures the level of allergy antibodies in the bloodstream, its accuracy is not affected by the presence of antihistamines or other medications, requiring no preparation changes. However, patients must always consult with their prescribing physician before making any changes to a prescribed medication.