In Makena's case, doctors had for years widely prescribed the drug to help prevent pre-term births. KV merely paid another company to push an already existing and widely used drug over the bureaucratic hurdles to win approval, and bought the marketing rights for about $200 million.

But the FDA's announcement calls that investment into question, along with the system of granting orphan status. The agency said it would not enforce KV's market protections unless other drugmakers produced unsafe or substandard versions of the drug.

Asked to quantify the value of the agency's award of market protection — given its explicit statement not to enforce it — FDA spokeswoman Beth Martino declined to comment. She described KV's case as a "unique situation."

"For many years, this compound has been available to patients," she said.

Martino stressed that the FDA's enforcement actions are discretionary and that for now it has chosen not to take action against other suppliers in order to 'support access to this important drug."

Depending on the circumstances, Martino said, "we may revisit our decision not to take enforcement action."

CRITICS CHEER DECISION