The U.S. Food and Drug Administration’s accelerated approval program is meant to give patients early access to promising drugs. But how often do these drugs actually improve or extend patients’ lives?

In a new study, researchers found that most cancer drugs granted accelerated approval do not demonstrate such benefits within five years.

“Five years after the initial accelerated approval, you should have a definitive answer,” said Dr. Ezekiel Emanuel, a cancer specialist and bioethicist at the University of Pennsylvania who was not involved in the research. “Thousands of people are getting those drugs. That seems a mistake if we don’t know whether they work or not.”

The program was created in 1992 to speed access to HIV drugs. Today, 85% of accelerated approvals go to cancer drugs.

It allows the FDA to grant early approval to drugs that show promising initial results for treating debilitating or fatal diseases. In exchange, drug companies are expected to do rigorous testing and produce better evidence before gaining full approval.