You should report any abnormal drug reaction to your doctor. They will want to determine the underlying cause of your symptoms in order to know whether your reaction is allergic or nonallergic.

According to the World Allergy Organization (WAO), if you react to a drug with a noticeable symptom, there is about a one in 10 chance that it is an allergic reaction. It is important to know whether your reaction is allergic or nonallergic because an allergic reaction may develop into life-threatening anaphylaxis in the future.

A nonallergic reaction might be a side effect of the drug. A side effect is any secondary action of the drug that might occur in a healthy person. These reactions can be either adverse or beneficial. Most side effects are known before a drug is prescribed. (Your doctor should tell you about any known side effects before prescribing a medication).

Sometimes, a lower dose can reduce or eliminate any negative side effects.

A nonallergic reaction can be an idiosyncratic (unusual and unpredictable) reaction. This can occur after your first exposure to a drug. Idiosyncratic reactions are not a typical side effect, and are often due to a genetic or metabolic abnormality.

In some cases, your reaction to a drug may closely mimic an allergic reaction. This is called a pseudoallergy or sensitivity. In some cases, this is a known side effect of a medication. This can occur during first use of a drug. For example, many people who take narcotic pain relievers such as codine experience hives.

The first step in the diagnosis of drug allergy is a complete physical examination. Your doctor will want to know if you have other allergies or a family history of allergies. He or she will also want to know how long you were using the drug before your reaction began and whether you had used the drug before. You will be asked to describe your symptoms in detail.

If possible, see your doctor while you are experiencing the reaction to a drug. This will help your doctor make a diagnosis. If your doctor suspects a drug allergy, they can perform several tests to confirm a diagnosis.

Skin Tests

For some drugs, an allergy skin test may determine whether or not you are allergic to a substance. Depending on the drug, a doctor might perform a skin-prick test or intradermal test.

During a skin-prick test, the doctor injects a small amount of the drug into the skin—usually the back or forearm. If you are allergic, you will develop redness, a bump, or other noticeable skin inflammation.

Intradermal tests can test for allergic reactions to penicillin and some other antibiotics. During these tests, the doctor injects a small amount of the allergen just under the skin, and monitors the site for a reaction.

Blood Tests

Blood tests can determine whether you are allergic to certain drugs. While they are not as accurate as other test methods, a doctor might choose to do a blood test if there is concern that you will have an anaphylactic reaction to the drug. Because the blood is tested outside your body there is no risk of an allergic reaction.

You may experience some pain at the site where the blood is drawn. A blood test can detect allergies to only a few specific drugs, such as some antibiotics, muscle relaxants, and insulin.

Provocation Tests

In a provocation test, increasing doses of the drug are given at planned intervals. You may take the drug orally or under the skin. Any reaction indicates possible allergy or sensitivity to the drug. If the reaction is mild, or if there is no reaction, the drug may be a safe treatment for the patient. The risks of provocation tests include a severe reaction, potentially even anaphylaxis. This test is used only in special cases and is usually performed only at specialized allergy centers.