Q and A


How are the words ‘rare’ and ‘common’ defined for side effects?


The EMA (the European drug regulatory agency) has very specific definitions for words relating to frequency of side effects.

This is a good question because the words are often used wrongly.

This is shown in the table below: ie

  • Very common affects more than 1 in 10 people – ie the risk is 10% or higher
  • Common affects between 1 in 100 and 1 in 10 people – ie risk is 1% to 10%
  • Uncommon affects between 1 in 1,000 and 1 in 100 people – ie risk is 0.1% to 1%
  • Rare affects between 1 in 10,000 and 1 in 1,000 people – ie risk is 0.01% to 0.1%
  • Very rare affects less than 1 in 10,000 people – ie risk is less than 0.01% (This includes isolated reports, or where the absolute risk is not known or can’t be quantified. For example, if identified through post-marketing safety surveillance and the frequency cannot be estimated from the available data).
Term Numerical rate Percentage rate
Very common More than 1 in 10 10% or higher
Common 1 in 10 – 1 in 100 10% – 1% 
Uncommon 1 in 100 – 1 in 1000 0.1% to 1%
Rare 1 in 1000 – 1 in 10,000 0.01% to 0.1%
Very rare Less than 1 in 10,000 Less than  0.01%


  1. Josh Peasegood

    Hi Kaileen, what you have described is right. Limits are set to report side effects. When 10 out of 100 report the same side effect, the effect is listed as ‘common’. This means 10% of people will experience this side effect while on medication. ‘Very rare’ is a term to report side effects that have only been reported in isolated events. It is possible that the medication may not be a side effect but cause of the effect cannot be determined. This is why it is listed as ‘very rare’. These events can happen in as few as 1 in 10000 people.

  2. Kaileen

    What percentage of complaints makes a side effect considered a side effect? If 10 people in 100 complain of the same thing is that listed a side effect? I know there is a specific percentage of people that must experience a side effect for it to be technically listed with a medication.

  3. Josh Peasegood

    Hi Budy, these are words that are not routinely used to define side effect likelihood. Often the words are used more in casual language to remove numbers from conversation. You can almost equate these terms to the question for which you have posted. Often meaning very common, hardly ever meaning rare, and seldom meaning very rare. This isn’t a hard and fast rule and there is variability to how these words are used.

  4. Budy

    What about: hardly ever, often & seldom (or other freq adverb)?

    Thanks before

  5. Roy Trevelion

    Hi 2, Thanks!

  6. 2

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