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Many people think they have a drug allergy, but they don't -- they've had a bad reaction to the drug. Only 6% to 10% of all drug reactions are drug allergies.

Does it matter? It does. Here's how to tell the difference.

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Understanding Drug Allergies

When you're allergic to a drug, your immune system overreacts. It treats the medication like a dangerous foreign invader.

When your body senses the drug, it sends out special cells. They release chemicals to drive the substance out of your body. These chemicals can cause hives, swelling in your face, and tightened airways.

You won’t have an allergic reaction the first time you take a drug. Your body has to recognize the drug, which happens when you take it the second time. By then, your body is primed to attack.

Non-Allergic Drug Reactions

In a non-allergic reaction, your immune system isn't the cause of the reaction. Instead, the drug may be interacting with another drug you’re taking. Or it may have an unexpected effect on your body. You can have a bad reaction to a drug the very first time you take it.

Non-allergic reactions to drugs can be just as dangerous – or as mild -- as true allergies. They can even cause many of the same symptoms:

  • Common side effects, like upset stomach, vomiting, or headache
  • Hives
  • Shock

Some common drugs that tend to trigger non-allergic drug reactions include:

  • Aspirin and other NSAID drugs, like Advil or Motrin (ibuprofen), which may cause hives or trigger asthma
  • Dyes used in X-rays and CT scans
  • Drugs for heart disease and high blood pressure, which cause non-allergic cough and swelling in the face and lips
  • Pain relievers like codeine or morphine

Some drugs -- like aspirin -- cause non-allergic reactions in some people and true drug allergies in others.

What to Do for a Drug Reaction

If you have a non-allergic adverse reaction to a drug, you can:

  • Ask your doctor about changing your dose. Simply taking less can sometimes reduce many side effects. 
  • Switch drugs. You may only have an adverse reaction to one drug. Another one in the same class might work well and not cause problems.
  • Protect yourself. If you know you react badly to a drug, take care. Make sure that all your health care providers know about it. You may want to get medical ID jewelry -- like a bracelet or necklace -- so that others can know about your drug reactions in an emergency.

WebMD Medical Reference

Reviewed by David T. Derrer, MD on December 05, 2014

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