Pat Hastings/Shutterstock. They may look pure and natural, but herbal supplements often don't contain what you expect.

If you buy illegal drugs on the street, you probably expect impurities. However, when it comes to herbal supplements bought at the largest chains in America, you might think the product will be a bit closer to what you are paying for. If so, you're probably wrong.

The New York State attorney general's office tested store-brand supplements from Wal-Mart, Walgreens, Target and GNC. Not only did many of the tablets contain potential allergens not listed in the ingredients, some showed none of the supposed active ingredient at all.

The Office of the Attorney General has sent cease-and-desist letters to the four giant chains instructing them to stop selling a range of products. In the letter sent to GNC, for example, the office states, “The supplements tested included Ginkgo Biloba, St. John's Wort, Ginseng, Garlic, Echinacea and Saw Palmetto. By using established DNA barcoding technology, analytic testing disclosed that 5 out of 6 types of dietary supplement products tested were either unrecognizable or a substance other than what they claimed to be, and therefore constitute contaminated or substituted products.”

To be fair, GNC's products were not all junk. Their garlic pills contained real garlic. Tests for Saw Palmetto proved mixed. As for the other four products, if you were lucky, you might get a member of the Allium genus (onion, garlic or leek) or something else generally harmless like rice. In many cases, testing revealed no plant DNA at all, raising disturbing questions about just what the pills actually contain. In some cases, the plants that were found are known allergens, including wheat.

There were some differences across stores. Target's echinacea pills, for example, mostly contained echinacea, but ginkgo biloba and St. John's Wort could not be detected in the products sold under those names in any of the stores.

The Attorney General is particularly concerned about the fact that previous studies have shown similar results, yet the companies seem to have taken no action.

Three chains will stop selling the supplements within New York state, while Target—despite having the least bad record of the four—will take them off the market nationwide.

The industry lobby group, the Council for Responsible Nutrition, has criticized the testing process, alleging that DNA can be removed or damaged in the manufacturing process. They fail to explain why DNA could be found from plants that were not supposed to be there in the first place.

While there is evidence to support the effectiveness of some of these herbs when customers actually get what they pay for, in other cases the benefits are doubtful. The fact that millions of people believe these supplements have made them better in the past is a tribute to the power of the placebo effect. Still, at least when you buy a loosely regulated product from a multi-billion dollar business it's not coming from “Big Pharma.”