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The first cases of liver problems linked to a popular weight loss supplement called OxyElite Pro began in Hawaii last May. But it wasn’t until four months later that the Food and Drug Administration , the agency responsible for getting dangerous products off the market, learned of the outbreak, and the product was finally recalled in November.

Flaws in the way that dietary supplements are monitored and reported are causing potentially life-threatening delays in how long dangerous products linger on store shelves, said Dr. Pieter Cohen, an assistant professor at Harvard Medical School who has studied supplement safety problems. They are also, he said, setting the stage for similar episodes to occur again.

Writing in a recent editorial in The New England Journal of Medicine, Dr. Cohen said that one of the more significant problems in such instances is that many cases of harm are reported to doctors and poison control centers but are either delayed in reaching the F.D.A. or do not make it to the agency at all.

“We’ve all been working under the premise that once a dangerous supplement is identified, the F.D.A. will swiftly remove it from the market,” he said. “But what I’ve come to realize over the last several years is that dangerous supplements remain on store shelves sometimes indefinitely or much longer than they should be.”

Many doctors who suspect a supplement has injured a patient neglect to submit a timely report to MedWatch, the online portal that the F.D.A. maintains to help identify clusters of injury linked to harmful products, Dr. Cohen said. He noted that delays occurred in 2008, when local health departments determined that more than 200 people had suffered selenium poisoning from a brand of multivitamin. And it happened in 2011, when the Department of Defense removed a workout supplement from military bases that contained a powerful stimulant known as DMAA.

“It took the F.D.A. an additional 16 months after the military ban to gather sufficient safety data from MedWatch to alert consumers about DMAA’s risks,” he wrote. In all of these cases, he added, “MedWatch reports were irrelevant.”

The F.D.A. has been stepping up efforts to make doctors more aware of the MedWatch system and to encourage them to use it to report promptly any serious reactions to drugs, supplements and other products, said Daniel Fabricant, the director of the division of dietary supplement programs in the agency’s Center for Food Safety and Applied Nutrition.

Even so, he said, doctors sometimes neglect to include critical information in their reports. Often left out are details about the product and its label, the results of any blood work or diagnostic tests, and whether the patient was using any other drugs or supplements, which can make follow-up difficult. Linking a single supplement that might contain dozens of different ingredients to an illness requires a forensic investigation that takes time, he said.

“I certainly understand and share Dr. Cohen’s concern,” Dr. Fabricant said. “But I do think the system has been useful and successful and instrumental in the agency taking action against products that were unsafe.”

Before joining the F.D.A. in 2011, Dr. Fabricant was a top executive at an industry trade group, the Natural Products Association. The F.D.A. recently announced that Dr. Fabricant is leaving the agency this month to return to the trade group as its chief executive.

Dr. Cohen said that many doctors simply do not find the F.D.A. or MedWatch very helpful in dealing with patients who have ingested a tainted product. So instead they turn to poison control centers, which offer medical advice in cases where a patient ingested something toxic. Indeed, in a report last year, the Government Accountability Office noted that from 2008 to 2010, more than 1,000 adverse events related to dietary supplements were reported to poison control centers – but not to the F.D.A.

“Because data are not routinely shared between the centers and the agency,” Dr. Cohen wrote, “the F.D.A. did not have access to these reports.”

Critics of the $32 billion a year supplement industry argue that the F.D.A. is hampered by a federal law that, they say, was written largely to protect the industry, the 1994 Dietary Supplement Health and Education Act.

The law, which gives companies much leeway in how they manufacture and advertise supplements, allows them to put products on the market without F.D.A. approval. As a result, the agency has little power to stop tainted products from landing on store shelves in the first place, even though research suggests that adulteration and mislabeling are widespread.

But industry representatives like Steve Mister, the president and chief executive of the Council for Responsible Nutrition, a supplement trade group, said the problem is not the law but an overstretched F.D.A., which has not aggressively pursued the industry’s bad apples. “The F.D.A. is grossly underfunded, it needs more resources, and Congress can do that,” he said.

Dr. Cohen said that at the very least, Congress should require that supplements carry information about side effects on their packaging.

“Right now, even when we know that a supplement has side effects, there’s no requirement that it appear on the label,” he said.

Dr. Cohen also called for a nationwide surveillance system in which the F.D.A. would working with poison control centers, local health departments and the Centers for Disease Control and Prevention. The system would include a rapid response team made up of toxicologists and other experts who could investigate reports and contact doctors and patients. A pilot program based on this idea was carried out in San Francisco in 2006 and shown to be successful, but it ended after one year because funding ran out.

Such a system, Dr. Cohen said, would help the kind of patients he has treated as a general internist at the Cambridge Health Alliance in Massachusetts.

“I had patients who would end up hospitalized – sometimes psychiatric hospitalization, sometimes kidney failure – because they were taking these weight loss supplements that they hadn’t been telling us about,” he said. “Since then, this has been a real focus of mine. What are these lessons I’ve learned from my clinical practice telling us about the sale of supplements throughout the United States?”