What is an FDA complete response letter?
When a drug or medical device is reviewed by the U.S. Food and Drug Administration (FDA), the company that created it receives a letter with the FDA's response. Before 2008, those letters were simply "approvable" or "not approvable" letters, which made it very clear to everyone paying attention to what would likely happen next. An "approvable" letter meant that it was close to being approved but needed more data or a small change. A "not approvable" letter meant that there was no way that drug was getting approval with the application that had been submitted.
Today, there's something quite different called the "FDA complete response" letter. Like the "approvable" and "not approvable" letters, an FDA complete response letter means that a drug or medical device hasn't been approved by the FDA, but the extent of the denial isn't evident simply from the letter type itself. It may be that the drug needs to have more easily obtainable information submitted, or it may mean that the drug requires years more of studies before it can stand even a chance at approval.