Patients may not realize it, but many of the prescriptions they get from their doctors are "off the books" -- that is, for uses not approved by the Food and Drug Administration. It's legal, and a new study shows it's an accepted part of medical practice, with more than 20 percent of all prescriptions being written for what are called "off-label" uses.

Off label use is common knowledge within the medical profession. But what hasn't been known is how commonly it's done. Randall Stafford of Stanford University was one of three researchers who studied data collected from 35,000 physicians.

Stafford says that 21 percent of prescription drug use in the United States is for treatments other than those the drugs were first approved by the FDA. "That equates to approximately 150 million uses of these drugs in a single year," Stafford said.

But what surprised Stafford and his colleagues was the rarity that the off-label uses were backed up by a consensus among physicians. Part of the discrepancy, he says, is a lack of strong scientific evidence for off-label uses.

Drug companies are prohibited from sending salespeople into doctors' offices to urge them to consider off-label uses, hoping to drive up a drug's sales. But doctors still learn about off-label uses from other doctors working for drug makers.

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