It wasn't until 1962 that Congress ordered the FDA to review all new medications for effectiveness. Thousands of drugs already on the market were also supposed to be evaluated. But some manufacturers claimed their medications were "grandfathered" under earlier laws, and even under the 1962 bill.

Then, in the early 1980s, a safety scandal erupted over one of those medications. E-Ferol, a high potency vitamin E injection, was linked to serious reactions in some 100 premature babies, 40 of whom died.

In response, the FDA started a program to weed out drugs it had never reviewed scientifically. Yet some medications continued to escape scrutiny.

Sometimes, the medications do not help patients. In other cases, the FDA says, they have made people sicker, maybe even killed them. This year, for example, the FDA banned injectable versions of a gout drug called colchicine after receiving reports of 23 deaths. Investigators found the unapproved drug had a very narrow margin of safety, and patients easily could receive a toxic dose leading to complications such as organ failure.

Critics say the FDA's case-by-case enforcement approach is not working.

"The FDA does not appear to have a systematic mechanism to report these drugs out," said Jon Glaudemans, senior vice president of Avalere Health, a health care industry information company, "and there doesn't seem to be a systematic process by which health insurance programs can validate their status. And everyone is pointing the finger at someone else as to why we can't get there."

In most cases, doctors, pharmacists and patients are not aware the drugs are unapproved.

"Over the years, they have become fully entrenched in the system," said Patti Manolakis, a Charlotte, N.C., pharmacist who has studied the issue. Only a few unapproved drugs are truly essential and should remain on the market, she added.

Tackling the problem is made harder by confusing — and sometimes conflicting — laws, regulations and responsibilities that pertain to different government agencies.

Medicaid officials said their program, which serves the poor and disabled, is allowed to pay for unapproved drugs until the FDA orders a specific medication off the market. But that can take years.

Compare that with Medicare, the health care program for older people.

Medicare's prescription program is not supposed to cover unapproved drugs. Medicare has purged hundreds of such medications from its coverage lists, but continues to find others.

It might be easier to sort things out if the FDA compiled a master list of unapproved drugs, but the agency hasn't. FDA officials say that would be difficult because many manufacturers do not list unapproved products with the agency. Yet, the AP found many that were listed — a possible starting point for a list.