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Side Effect or Allergy? How to Tell the Difference

man with medication
By Frieda Wiley, PharmD, RPhApril 01, 2019

As a pharmacist, many of my patients have told me they were allergic to a certain drug, but the reaction they described was actually a side effect. It’s important to know the difference because allergic reactions—especially those that are life-threatening—can mean it’s not safe for you to take a certain medication or group of similar medications. Side effects, though – unless they are severe or life-threatening, are not necessarily signalling that you need to quit taking the medication. Many side effects are mild and manageable, and may even go away with time as your body adjusts to the medication. So how can you tell the difference between allergies and side effects?

Allergies: Your Immune System Acting Like a Drama Queen

An allergy occurs when your body’s immune system comes into contact with something it doesn’t recognize and overreacts, or “freaks out.” In other words, your immune system starts acting like a drama queen and making mountains out of mole hills. Symptoms of an allergic reaction can vary range from something as a simple as a light rash to something much more serious and potentially life-threatening such as trouble breathing and your throat closing up (anaphylaxis). In most cases, you’ll know almost right away if your medication is causing a life-threatening reaction worthy of an Epi-Pen shot and an ER visit. Other reactions may take as many as a few weeks to show up. Allergies usually go away when whatever is stressing your immune system is removed (e.g., You avoid mold, you stop taking the drug). Other signs and symptoms of allergies include:

  • Itching
  • Itchy watery eyes
  • Redness
  • Hives
  • Wheezing

Side Effects: The Body Acting Like a Diva

Medications can sometimes unleash your body’s inner diva by demanding that you give your body some extra attention when it responds to a medication in an unpleasant way. But one of the key differences between an allergy and a side effect is that your immune system dictates the allergic response, but it plays no role in side effects. Instead, side effects are consequences of the way the medication works. As an example, let’s look at how water pills might prompt the side effect of muscle cramps. Water pills make you “pee off” water, right? Losing all of that water can throw off your electrolyte levels, or the amount of sodium and potassium in your body; and when those levels get too low, our muscles cramp.

Like allergies, side effects can range from mild to severe, but unlike allergies, side effects may go away as your body adjusts to the medication after a few days or weeks. Everyone is different, so there is no hard and fast rule on how long it takes for your side effects to subside. But it's a good sign if you notice your signs and symptoms improving each day.

Examples side effects include:

  • Nausea
  • Dizziness
  • Palpitations
  • Headaches
  • Drowsiness
  • Confusion

Sometimes, medication side effects mirror allergies, so it can be difficult to tell the difference between them. For example, swelling or itching can indicate an allergic reaction in many cases, but these are also side effects of certain medications. So, pay attention to the medication guides that come with your prescription medications - they explain what side effects to expect and when you should seek medical attention for an allergic reaction. These handouts are definitely helpful, but if you still have questions, ask your pharmacist. 

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