But Erin Fox, who tracks supply levels for a broader range of drugs at the University of Utah, said once a drug became scarce, it tended to stay scarce. The university’s Drug Information Service was actively tracking 282 hard-to-find products by the end of the third quarter of this year, a record.

“The shortages we have aren’t going away — they’re not resolving,” she said. “But the good news is we’re not piling more shortages on top.”

In 2011, prompted by emotional pleas by cancer patients and others who said the drug shortage was threatening lives, President Obama issued an executive order requiring drug makers to notify the F.D.A. when a shortage appeared imminent. The agency also loosened some restrictions on importing drugs, and sped up approvals by other manufacturers to make certain medicines.

A law passed this summer contains several provisions aimed at improving the situation, including expediting approval of new generic medicines and requiring the agency’s enforcement unit to better coordinate with its drug-shortage officials before it takes action against a manufacturer.

Ralph G. Neas, the chief executive of the Generic Pharmaceutical Association, said fixing the drug shortage was complex and would take time, but was a top priority. “One shortage is one shortage too many,” he said. “One patient not getting a critical drug is one patient too many.”

Federal drug officials trace much of the drug shortage crisis to delays at plants that make sterile injectable drugs, which account for about 80 percent of the scarce medicines. Nearly a third of the industry’s manufacturing capacity is not running because of plant closings or shutdowns to fix serious quality issues. Other shortages have been caused by supply disruptions of the raw ingredients used to make the drugs, or by manufacturers exiting the market.

Some people have accused the F.D.A. of causing the shortages, saying overzealous enforcement and poor communication have led plants to close needlessly or to slow production. Others have cited economic factors, like market pressures and reimbursement policies that have set prices so low that some companies have stopped making certain drugs. Earlier this week, several Democratic members of Congress asked the Government Accountability Office to investigate whether the practices of so-called group purchasing organizations, which buy drugs on behalf of hospitals, was contributing to the shortage.