Do Rare Side Effects Really Happen?

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 side 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.”


Suzanne B. Robotti

Suzanne B. Robotti

Suzanne Robotti founded MedShadow Foundation in 2012. Learn more about Su and her mission.


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