Drug allergies are the result of an exaggerated immune system response to a drug or class of drugs. Those who are allergic to a medication are likely to have allergies to other medications of a similar class. For example, those with allergies to penicillin are likely to be allergic to other antibiotics such as cephalosporins.

Drug allergies are more common in adults than in children. Anyone who has a history of allergies – including hay fever and eczema – or asthma is at higher risk of developing a drug allergy. Having a family history of drug allergies is a risk factor as well. Other risk factors for drug allergies include frequent exposure to a drug or taking large doses of the drug.

The method in which a drug is administered is also a factor. Drugs that are given intravenously or through injection are more likely to cause an allergic reaction than those taken orally. This is because drugs that are injected or given by intravenous route are sent directly into the bloodstream. 

Those at risk for allergies to multiple medications include individuals with immune disorders such as AIDS or lupus, and those who receive neutral protamine Hagedorn (NPH) insulin. These reactions are not usually life-threatening. 

Drugs known to trigger allergies include:

• Beta-lactam antibiotics
• • Penicillin
  • Amoxicillin
  • Ampicillin
  • Dicloxacillin sodium
  • Nafcillin sodium
  • Penicillin V
  • Penicillin G
    
    
• Cephalosporin antibiotics
• • Cephalexin monohydrate
  • Cefprozil
  • Cefixime
  • Cefpodoxime proxetil
  • Cefaclor
  • Cefuroxime axetil
    
    
• Sulfa antibiotics
• • Co-trimoxazole
    
    
• Macrolide antibiotics
• • Erythromycin
    
    
• Anticonvulsants
• • Phenytoin
  • Carbamazepine
  • Valproic acid
  • Lamotrigine
    
    
• Insulins
• • Pork insulin
  • Beef insulin
    
    
• Analgesics
• • Codeine
  • Morphine
  • Aspirin and other NSAIDs (e.g., ibuprofen, indomethacin)

Anyone who suspects that they are allergic to a medication should see a physician. Only a physician is qualified to determine whether allergy symptoms are caused by a medication or an underlying condition.