Billions of prescriptions for generic drugs are filled in the U.S. annually. The FDA tests only a few dozen of them every year, its own records show, and it has largely dismissed warnings about contaminants, irregularities and other issues.

It’s a fundamental tenet of health care in America: Generic drugs are just as safe and effective as brand-name ones. The only difference is the price.

“The same high quality, strength, purity and stability,” the Food and Drug Administration assured the public years ago as factories started to flood the market with their own, cheaper versions of commonly used drugs, from antibiotics to cancer treatments.

But the agency stakes that promise on a risky gamble.

It doesn’t routinely test generics for quality concerns or to see if they’re working as effectively as brand-name medications. Instead, the agency heavily relies on drug companies, often in countries as far away as India and China, to do their own testing and to report any problems.

Yet the FDA largely dismissed the warnings and has only sporadically tested a sampling of generic drugs, which now account for about 90% of prescriptions in the United States. That means the government can’t always say which ones may be compromised or how often that happens. And patients can’t make informed choices about which drugmakers to depend on.