The FDA is everywhere these days, or so it seems. Sellers of dubious medical devices claim in late-night TV advertisements that their products are "registered with the FDA". Some vitamins and homeopathic remedies, many with doubtful benefits, boast on their labels that they are "FDA approved."

In the 1990s, the agency was widely criticized as taking too long to approve new drugs, so Congress passed legislation speeding up drug reviews. Then in 2004, Merck withdrew its painkiller Vioxx after studies showed the pill increased the risk of heart attacks and strokes.

Critics lambasted the agency, saying it approved medicines too easily and for months delayed issuing warnings about potential drug risks. The agency now issues a blizzard of risk alerts, but sifting through this information can be difficult. And it can be tough to figure out which products have the agency's crucial stamp of approval—or what bits of product information have actually been vetted by federal regulators.

For instance, the drug information sheets that pharmacists routinely give to patients along with their prescription medicines—the sheets are sometimes printed on receipt paper—seem like official government pronouncements.

But the FDA has no role in writing or overseeing most of these sheets. Public Citizen, a watchdog group, has found that some drug information sheets are dangerously wrong or misleading.

The FDA is the federal agency charged with overseeing the safety of drugs, medical devices, food, cosmetics and many other health-related products. But although the agency has regulatory authority over nearly 25 percent of the nation's economy, it is important to understand what it regulates closely and what it doesn't.

For example, the FDA does almost nothing to ensure that dietary supplements work as advertised. Only when supplements are proved to be unsafe or to contain regulated substances can the agency take action.

In the past year, the agency has seized thousands of Asian herbal supplements promoting virility because they contained the active ingredient in Viagra, a regulated substance. Similarly, the FDA exempts over-the-counter homeopathic remedies from its testing. So any reference to the FDA on dietary supplements or homeopathic medicines generally does not mean that the agency has found the products to be safe or effective.

For drugs, the FDA's role is more complicated. In most cases, the FDA demands that manufacturers seeking to sell drugs in the United States, either over the counter or prescription, prove that they're safe and effective. But there are exceptions, and these can be hard to spot. A surprising number of older prescription medicines have been around for so long that they have never been vetted by the agency. Officials have promised for decades that they would catch up with these products. But dozens, perhaps hundreds, still remain.