The incentive to discover a useful new drug is clear. Patents and exclusive marketing rights granted by the Food and Drug Administration enable drug companies to profit from a monopoly. The monopoly prices allow them to earn enough to recover the cost of developing new drugs — the ones that succeed and the attempts that fail — as well as profit. There is no comparable incentive to investigate the use of existing drugs as solutions for other health issues, what the University of Michigan law professor Rebecca Eisenberg termed “the problem of new uses.”

Despite the limited financial incentives and investment, over time, some valuable new uses of previously developed drugs have been found. Sometimes new uses are discovered by accident or through publicly sponsored studies. The diabetes drug metformin appears useful in treating breast cancer. Thalidomide, the morning sickness treatment that caused birth defects, is now approved for the treatment of multiple myeloma, a type of blood cancer. In 2012, researchers experimenting on mice discovered that a cancer treatment drug showed encouraging early signs of effectiveness against Alzheimer’s disease.

But there is little incentive to evaluate an existing drug for additional uses to the extent required for F.D.A. approval. Though obtaining that approval would be cheaper than developing a new drug, drug companies can encourage off-label prescribing with even cheaper studies than the F.D.A. requires. Additionally, more rigorous testing of drugs for new uses might illuminate harms, hampering drugs’ marketability rather than helping it. With little to gain and a lot to lose, companies aren’t eager to invest in new-use studies.

“Why bother spending millions to get F.D.A. approval of a new use for a medication, when one small study, some key opinion leaders and an army of sales representatives can do the trick instead?” Dr. Gellad said.

Once a drug’s market exclusivity period expires and generics enter the market, companies have even less incentive to test their products for new uses. Generic competition pushes down prices of drugs, which is good for patients who could not otherwise afford them. But at those lower prices, companies cannot recoup the costs of clinical trials to test drugs for new indications. What’s in it for them?