Drugmakers are slow to prove medicines that got a fast track to market really work

When there's an urgent need, the agency can grant what's known as an accelerated approval for a drug, like Clolar, based on preliminary evidence.

In the FDA's approval of Clolar, the agency noted there were no gold-standard clinical studies to show whether the drug prolonged patients' lives or improved their health.

In exchange for the accelerated approval, the drugmaker, Genzyme, agreed to do definitive follow-up studies to prove that Clolar really worked. But while that work was pending, the company was free to keep selling the drug.