More than half of all cancer drugs recently approved by the Food And Drug Administration may not help patients live longer, according to a new study by an Oregon doctor.

The FDA approves cancer drugs in different ways.

The most comprehensive way requires manufacturers to show their drug actually extends a patient’s life. But they can also get accelerated approval by showing their drug shrinks tumors or stops them growing.

Dr. Vinay Prasad of Oregon Health and Science University looked at 54 drugs approved between 2008 and 2012. He found only five showed evidence of improved survival and 18 showed no effect on survival.

Prasad thinks it is okay for the FDA to approve drugs that only shrink tumors, “But I do think it’s important for patients and for the public that in a reasonable period of time these drugs should  followed up to make sure that they are not false leads — that they actually do what we think they do, which is make people live longer,” he said.

In a statement, the FDA says it’s widely accepted that the benefit of a drug can be demonstrated many ways, not just by survival.

The study is reported in JAMA Internal Medicine and is co-authored by Dr. Chul Kim of the National Cancer Institute.