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Do Rare Side Effects Really Happen?

Suzanne B. Robotti
Suzanne B. Robotti Executive Director

You hear about them in ads, but you never know if unusual side effects might happen to you

When a side effect is listed as “common,” what does that mean? What are the odds of a “rare” side effect happening to me? It was news to me that these terms are actually very specific. The World Health Organization (WHO) defines categories as:

  • Very common means 1 in 10 — 1 out of every 10 people (or more) taking that medicine will experience that side effect.
  • Common means more than 1 in 100 — between one in 10 and one in 100 people are affected
  • Uncommon means more than 1 in 1,000 — between one in 100 and one in 1,000 people are affected
  • Rare means more than 1 in 10,000 — between one in 1,000 and one in 10,000 people are affected
  • Very Rare means more than 1 in 10,000+ — fewer than one in 10,000 people are affected

I found these definitions to be comforting. After all, a rare effect that happens to 1 in 1,000 to 10,000 seems safe. Then I pulled out my calculator. As an example, Crestor is one of the most prescribed drugs in America, through June 2015. It “rarely” causes liver damage (among other things). 21 million prescriptions for Crestor were written from July 2014-June 2015. Doing the math reveals that it’s possible that between 2,100 and 21,000 people in the US suffered new onsets of liver damage from taking Crestor each year.

Then I realized that I need to look more closely at rare effects listed as “common.” Looking at the package insert for statins, common side effects are diarrhea, upset stomach, muscle and joint pain and changes in some blood tests (I don’t even know what that means). With so many common side effects, I wonder how many statin-takers don’t experience any side effects?

The odds of having a common side effect is high and the likelihood of a rare side effect is long, but real. As my friend Tom says, “It’s all well and good if you’re not that 1 in 1,000 but if you are, that really stinks.”

DISCLAIMER: MedShadow provides information and resources related to medications, their effects, and potential side effects. However, it is important to note that we are not a substitute for professional medical advice, diagnosis, or treatment. The content on our site is intended for educational and informational purposes only. Individuals dealing with medical conditions or symptoms should seek guidance from a licensed healthcare professional, such as a physician or pharmacist, who can provide personalized medical advice tailored to their specific circumstances.

While we strive to ensure the accuracy and reliability of the information presented on MedShadow, we cannot guarantee its completeness or suitability for any particular individual's medical needs. Therefore, we strongly encourage users to consult with qualified healthcare professionals regarding any health-related concerns or decisions. By accessing and using MedShadow, you acknowledge and agree that the information provided on the site is not a substitute for professional medical advice and that you should always consult with a qualified healthcare provider for any medical concerns.

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