Classification of Reactions to Medications



Before we can discuss the huge topical of medication allergy, we need to group reactions to drugs into one of 2 categories:

- Reactions which are common and predictable in any person. This would include expected side effects from medications, interactions between 2 medications that the person is taking, and reactions from using too much of the medication (overdose). This group represents the majority of all reactions to medications.

- Reactions which are unpredictable, and only occur in certain people. These reactions can include an unexpected side effect, medication intolerance, allergic reactions and other non-allergic immunologic reactions.

Allergy to Common Medications

People can experience allergic reactions to just about any medication, although some are more common than others. Here is a list of the most common medication allergies (or non-allergies, in some cases):

1. Penicillin (and all related antibiotics). About 1 in every 10 people reports a history of an "allergic reaction" to penicillin. It turns out that much less than 10% of those who think they are allergic to penicillin actually are. However, people with a true allergy to penicillin could have life-threatening anaphylaxis as a result, it is important to tell your doctor about your past reaction to the medication. Skin testing to penicillin can help determine if the past reaction was a true allergy or some other side effect.

2. Cephalosporins (and all related antibiotics). Severe reactions to cephalosporins are much less common than with penicillins. However, there is a small chance that someone with a true penicillin allergy could also react to cephalosporins, since the drugs are related. An allergist may be able to help determine if these antibiotics are safe for you.

3. Sulfonamides (including antibiotics, oral diabetes medications and some water pill diuretics). It is unclear whether these reactions are truly allergic or due to another immunologic process. There is no reliable test available to determine is a person is allergic to this class of medications.

4. Non-Steroidal Anti-Inflammatory Drugs (NSAID), including aspirin, ibuprofen and naproxen. This class of medications can cause allergic and non-allergic flares of hives/swelling, worsen asthma, and result in anaphylaxis. There is no reliable test available for most people with reactions to these medications.

5. IV Contrast Dye. This reaction is non-allergic but can result in anaphylaxis because the high concentration of the dye causes mast cells to release their contents, which mimics an allergic reaction. While there is no test available for reactions to IV contrast, most patients can take the dye safely by taking oral steroids and anti-histamines hours before the contrast is given. The contrast is usually given in a less concentrated form to these patients. Let your doctor know if you've had a past reaction to IV contrast before receiving it again.

6. Local Anesthetics. True allergic reactions to local anesthetics (novocaine, lidocaine) are extremely rare, and usually due to other ingredients in the medication, such as preservatives or epinephrine (present in the local anesthetic to make the medication last longer once it's injected). An allergist can perform testing to various local anesthetics and find one that works for almost everybody.

7. General Anesthesia. Some medications used during surgery are very common causes of true allergic reactions and anaphylaxis. If you think you experienced an allergic reaction during or shortly after surgery, an allergist may be able to help determine the cause.

8. Anti-Seizure Medications. Many medications used for treatment of epilepsy can cause non-allergic reactions as a result of certain enzyme deficiencies in the person taking the medication. Symptoms can include a rash, fever, body aches and hepatitis. There is no test available for this type of reaction.

Management of Adverse Reactions to Medications

1. Avoidance/Removal of the suspect medication. Most often, if a medication is causing a person to have an unpleasant reaction, the medication is stopped. Usually this is all that is needed to resolve the problem.

2. Treatment of any allergic or non-allergic symptoms. If the reaction is severe, such as anaphylaxis, or in Stevens-Johnson Syndrome/toxic epidermal necrolysis, emergency treatment may be required.

3. Consideration of testing by an allergist. An allergist may be able to help determine what medication caused the reaction and if there is testing that can be accomplished.

4. Determination of a need for current or replacement medication. If the person still has an infection, for example, another antibiotic may be required. A physician can determine a safe alternative medication in this instance.

5. Consideration of desensitization. In some unique cases, a very specific medication is needed. In most forms of true medication allergy, a person can be desensitized to the medication by giving very small amounts initially (orally or intravenously) with increasing doses over many hours. This is usually done in a hospital given the chance of life-threatening anaphylaxis, and should only be done under the supervision of an allergist.