Drugs can cause a variety of common allergic reactions of which a rash is the most common. Antibiotics and aspirin-like medications are the most common causes. Drug allergies can be hereditary—people are more likely to have an allergic reaction to a medication if one of their parents has a drug allergy. However, that is usually not sufficient reason to avoid a drug. Diagnosis of drug allergy hinges on the timeline of drugs taken and an accurate description of the event. Allergy skin testing and drug challenge are sometimes performed. When sensitivity is know, drug desensitization (especially for aspirin allergy) is performed but usually avoidance is recommended.
Specific drug allergies:
Penicillin Allergy: Most people who believe they have a penicillin allergy “pass” an allergy test to the drug and can take penicillin. Some were never allergic (symptoms attributed to the penicillin were caused by another condition) and others “outgrew” the allergy. A simple office based test for penicillin allergy is very reliable. It’s best to sort this out before the need develops. Patients allergic to penicillin are also sometimes told to avoid a somewhat related category of antibiotics (cephalosporins). If you can confirm that you’re not allergic to penicillin then you will decrease the risk from using the newer antibiotics, which are more expensive, sometimes more toxic, and may lead to bacterial resistance. This is especially important for women of child bearing age who have a history of penicillin allergy. Pregnant women who have silent Group B strep need to be treated and this is ideally with penicillin. Although treatment with the third generation cephalosporin is effective for Group B strep many women with a history of PCN allergy are instead treated with Vancomycin or Clindamycin (and Clindamycin resistance is an emerging problem with Group B strep). Dentists also often prescribe Clindamycin for penicillin-allergic patients but this alternative antibiotic may wipe out too many healthy intestinal bacteria leading to severe diarrhea (C. Dificile colitis).
Aspirin/NSAID Allergy: The mechanism of this allergy and the consequences vary. People who have an allergic reaction to aspirin are usually allergic to ALL NSAIDs (nonsteroidal anti-inflammatory drugs). Generally, Tylenol is tolerated well in most people allergic to aspirin. A subtype of NSAID allergy is the aspirin triad affecting patients with nasal polyps and asthma some of whom benefit from aspirin desensitization. The list of drugs that contain aspirin or NSAIDs is quite long and drugs are divided by categories based on their chemical structure:
- Acetic acids: Diclofenac, Etodolac, Indomethacin, Ketorolac, Nabumetone, Sulindac, TolmetinFenamates
- Fenamates: Meclofenamate, Mefenamic acid
- Oxicams: Piroxicam
- Propionic acids: Fenoprofen, Flurbiprofen, Ibuprofen, Ketoprofen, Naproxen, Oxaprozin
- Salicylates: Aspirin, Diflunisal, Magnesium salicylate, Salsalate
Local anesthetic allergy: Most of these reactions are not truly allergic but this must be sorted out. I have seen patients undergoing dental procedures including drilling without anesthesia because the think they are allergic. This is absolutely unnecessary and testing can always clarify the situation.
Contrast Allergy: Despite common belief, this has nothing to do with shellfish allergy. Allergy to contrast dye (used in CAT scans, MRIs and some other medical procedures) is quite uncommon. Severe reactions are even less common. The dyes used today are much less likely to cause allergic reactions. Some recent research suggests that patients can be tested for contrast dye allergy but it is not routinely done.