Frequency of drug side effect listed as "unknown"

Question

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.

Answer 1

A GUIDELINE ON SUMMARY OF PRODUCT [pharmaceuticals] CHARACTERISTICS (EUROPEAN COMMISSION, 2009)

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."