Dietary supplement fans got a big "buyer beware" warning this week when the New York attorney general's office ordered GNC, Target, Walgreens and Wal-Mart to pull a number of store-brand products from their shelves, following an investigation that found most didn't contain herbs listed on their labels. In some cases, the attorney general said the supplements didn't even identify potentially dangerous allergens.

It was the latest in a series of studies and investigations that have cast serious doubt on the safety and reliability of these products, which face laxer regulatory scrutiny compared to prescription drugs.

But none of that has changed the fact that Americans are nuts for dietary supplements, as you can see in the chart below (red bars indicate projected sales).

Sales in 2013 reached $13 billion, as more people turn to the supplements to boost their health and lose weight. One of their biggest boosters is syndicated TV host Mehmet Oz of "Dr. Oz" fame, even though "America's doctor," as he's also known, has gotten into trouble for pushing pills with little medical grounding.

When researchers take a closer look at the products, the results can be alarming. Researchers from a 2012 Inspector General's report found that 20 percent of the weight loss and immune system support supplements they purchased made illegal claims about their ability to treat and cure disease.

A year later, Harvard researchers found that between 2004 and 2012, there were 237 recalls of dietary supplements — accounting for more than half of FDA recalls of Class 1 drugs, which mean the products contain substances that can cause death or serious health problems. And in October, a JAMA study found most of supplements that were recalled for containing dangerous banned drugs were still available to consumers at least six months later.

There's a common public misperception that these products face the same rigorous oversight that pharmaceuticals receive from the FDA. They don't, thanks to a federal law that's been in place since 1994.

Manufacturers of dietary supplements are required to attest their products are safe and accurately tested, but unlike prescription drugs, they're not tested by the FDA before they go to market. The 2012 IG report recommended the FDA seek out this authority through legislation and to expand its limited surveillance of products once they're available to consumers.

The Council for Responsible Nutrition, a trade group representing dietary supplement manufacturers, often pushes back against studies that are critical of the industry. For example, they said the latest JAMA study proved that the vast majority of recalled items were successfully removed from the market. And the group slammed the New York attorney general's investigation as a "self-serving publicity stunt under the guise of public health," contending that his office's findings relied on flawed science.

Federal oversight of the industry has tightened—somewhat. Since 2007, manufacturers have been required to report anytime a consumer experiences a serious medical reaction (whether it's hospitalization or even death) to the FDA within 15 days. The agency received more than 6,000 reports between 2008 and 2011, according to a March 2013 Government Accountability Office report, with most of those coming from industry. However, the GAO said it believes these are probably under-reported because some consumers appear to report these events to poison control centers instead of the FDA.