The F.D.A. requires that companies verify that every supplement they manufacture is safe and accurately labeled. But the system essentially operates on the honor code.

Under a 1994 federal law, supplements are exempt from the F.D.A.’s strict approval process for prescription drugs, which requires reviews of a product’s safety and effectiveness before it goes to market.

The law’s sponsor and chief architect, Senator Orrin G. Hatch, Republican of Utah, is a steadfast supporter of supplements. He has accepted hundreds of thousands of dollars in campaign contributions from the industry and repeatedly intervened in Washington to quash proposed legislation that would toughen the rules.

Mr. Hatch led a successful fight against a proposed amendment in 2012 that would have required supplement makers to register their products with the F.D.A. and provide details about their ingredients. Speaking on the floor of the Senate at the time, Mr. Hatch said the amendment was based on “a misguided presumption that the current regulatory framework for dietary supplements is flawed.”

Critics say it is all too easy for dangerous supplements to reach the market because they are not subject to a review or approval process. Under current law, supplements are assumed to be safe until the authorities can prove otherwise. And in general, they are pulled from shelves only after serious injuries occur — which is not uncommon.

In 2013, for example, an outbreak of hepatitis that struck at least 72 people in 16 states was traced to a tainted supplement. Three people required liver transplants, and one woman died.

It is not only consumers. Hospitals have been affected, too. In December, an infant at a Connecticut hospital died when doctors gave the child a popular probiotic supplement that was later found to be contaminated with yeast. After the child’s death, the F.D.A. issued a warning to the public that reiterated its limited control over supplements.

“These products are not subject to F.D.A.’s premarket review or approval requirements for safety and effectiveness,” the F.D.A. stated, “nor to the agency’s rigorous manufacturing and testing standards for drugs.”