I'm looking at the side effects of Madopar, and all of them are listed as "unknown frequency".

How can frequency be unknown? Even if just one of a million patients got a certain side effect, that just means that frequency is 1 millionth, right?

The only reason I can think of, is that zero side effects occurred in the drug's clinical test, AND people contacted the company about their side effects AFTER the drug was already on the market. That seems unlikely, though.

I found this link but it's a bit of a wall of text.


1 Answer 1



In exceptional cases, if a frequency cannot be estimated from the available data, an additional category frequency ‘not known’ may be used.

Sometimes, knowing the number of cases of a drug side effects in a certain group of users is not enough to estimate the frequency of side effects in the entire population. For example, if only one person spontaneously reports a side effect, the reported frequency is known, but this may not be enough to estimate the actual frequency, because the producer does not know how many others experienced side effects.

When the reported frequency is low, the producer can't automatically claim the actual frequency is "low," so they may say it's unknown, but you can assume it is probably low, because...when the reported frequency is high, the producer can automatically estimate the actual frequency is also "high."

  • Thanks! Are you talking about reporting side effects inside of clinical studies, or after the drug is already on the market? Commented Apr 26, 2019 at 6:57
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    My above examples were about the time when the drug is already on the market. During clinical studies, the frequency is known because they actively ask every participant, but if there is only a small number of participants, then the estimated "real life" frequency can still remain uncertain. I mean, they can be various reasons why they can't reliably estimate the frequency. In that related short paragraph in the document you linked, it is said that the establishing the cause - side effect relationship may also be a problem.
    – Jan
    Commented Apr 26, 2019 at 7:03

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