The letters, many of which were already available online, detail why the regulators initially declined to approve some drugs. All eventually passed muster.

The Food and Drug Administration on Thursday published a database containing more than 200 letters to pharmaceutical companies detailing why the agency initially refused to approve their products, shedding light on an obscure and contentious aspect of drug development.

F.D.A. officials described the move as a form of “radical transparency.” But many of the documents were already available on the agency website, albeit often difficult to find and typically posted weeks after a drug’s approval.

The documents, referred to as “complete response letters,” are akin to rejection letters. Some outline problems with an experimental drug that cannot be remedied; those letters are never made public.

But the letters released on Thursday described the agency’s early concerns about drugs that were eventually approved.

In the past, some drug makers have informed investors that the agency has sent such a letter; that decision is often prompted by securities laws. But it has been left to the companies to explain what the F.D.A.’s worries were.

In a 2015 analysis, federal researchers discovered that the companies avoided publicly describing 85 percent of the agency’s concerns about safety and efficacy when a drug eventually gained approval.