Nonsense. That is not even close to the same. Yes, when tested on millions of patients instead of thousands, one might find new adverse effects. In reefing, we are lucky if the number of tank tests is more than 1 (or zero).
FYI...
To be approved for sale, pharmaceuticals typically get tested in at least 2 phase 3 trials, and the results of those trials, no matter what they are, are put into the package insert that patients and prescribers can see, including all health effect and concerns:
Here's the package insert for one of my drugs:
It has lots of pertinent sections on warnings, drug interactions, side effects, who should take it and who should not, and what to watch out for.
For example:
The following adverse reactions have been identified during post-approval use of sevelamer hydrochloride, which has the same active moiety as sevelamer carbonate: pruritus, rash, abdominal pain, fecal impaction, and uncommon cases of ileus, intestinal obstruction, and intestinal perforation. Appropriate medical management should be given to patients who develop constipation or have worsening of existing constipation to avoid severe complications.