We don't really have that for many present day medicines. We know they work, statistically speaking. But we don't know for all of them how they work, what the mechanism is. For some drugs this lags the discovery of the drug itself (historically: all drugs, but in the last 100 years fewer of them). For some drugs it simply hasn't happened.
https://en.wikipedia.org/wiki/Category:Drugs_with_unknown_me...
And for some it may never happen...