Major retailers in New York, including Walgreens, Wal-Mart, GNC, and Target, are clearing several popular herbal supplements from their shelves after the state attorney general issued subpoenas demanding proof of health claims on the bottles.

That bold challenge to an industry that generates nearly $100 billion annually has raised concerns - and hackles - nationwide.

For years, mainstream scientific journals have published studies showing that many popular herbal supplements, such as echinacea, confer few or no health benefits, and some, like St. John's Wort, can do harm.

For years, consumers have taken them anyway.

A 2007 National Health Interview Survey found that in one year, 17.7 percent of American adults took natural supplements, not including vitamins and minerals. When vitamins and minerals were included in a National Health and Nutrition Examination Survey that reviewed data from 2003 to 2006, the figure jumped to 53 percent of Americans.

The difference is notable, given that many doctors who do not recommend that their patients take herbal supplements do prescribe vitamins and minerals for certain conditions. It is standard practice, for instance, to prescribe folic acid for women contemplating becoming pregnant.

The recent uproar over supplements was ignited in early February when New York Attorney General Eric Schneiderman announced that state-commissioned studies on various herbal supplements found that 80 percent of the samples had no DNA traces of the purported main ingredient.

Several independent experts have said the results are misleading because the manufacturing process destroys DNA.

"People are imagining some thoughtful naturopath walking through fields, choosing flowers, and returning to his lab, carefully preparing it for you with a mortar and pestle," said Pieter Cohen, an internist at the Cambridge Health Alliance, an assistant professor at Harvard Medical School and an authority on herbal supplements.

"This is a huge manufacturing operation," he said, "producing massive quantities in the most inexpensive way."

Most herbal supplements, especially (but not exclusively) lower-priced varieties, are made from powders shipped in large containers from overseas.

By the time the St. John's Wort or Ginkgo biloba reaches the factory, it likely has been treated with solvents to dissolve chemical compounds from the plant, then heated to remove the liquid, Cohen said. Both processes can destroy DNA.

The Food and Drug Administration monitors production of herbal supplements, but not as closely as it controls prescription drugs and other pharmaceuticals.

Since there are no guidelines for the recipes manufacturers may use to make supplements, amounts of active ingredients can vary considerably.

In some cases, Cohen said, consumers could be better off getting less than more. St. John's Wort, for instance, can have serious interactions with prescription drugs that treat high blood pressure, HIV, depression, and asthma.

"The placebo effect," he said, "might be more safe than a whopping dose."

Tod Cooperman, president of, a firm that tests and rates health and nutritional products, agreed with Cohen that there is good reason to be concerned about the quality and content of herbal supplements.

Consumers who want to minimize their chances of ingesting something unintended should choose simpler formulations.

"A variety are sold that list 20 substances," Cohen said. "That's where we see much more dangerous stuff, pharmaceutical drugs masquerading as benign supplements, or novel stimulants never tested in humans."

It is not unusual for items labeled "natural" and "pure" to contain fillers, some benign, some not. Even with federal oversight, contaminants get into the products.

The Food and Drug Administration periodically visits supplement factories unannounced to see if the company is sticking to its recipes, keeping the facility clean, and putting the ingredients advertised on the label into the bottle.

"When the FDA does spot checks," Cohen said, "seven out of 10 companies aren't in compliance."

In June 2012, for example, the FDA issued a warning to the public after finding that the dietary supplements Reumofan Plus and Reumofan Plus Premium contained powerful prescription drugs, including methocarbamol, a muscle relaxant; dexamethasone, a corticosteroid, and diclofenac sodium, a nonsteroidal anti-inflammatory drug.

One of the few reliable ways to tell if a supplement contains only what it claims, Cooperman said, is to look for the U.S. Pharmacopeial Convention's (USP) seal of approval.

USP is a scientific nonprofit organization that sets standards for the identity, strength, quality, and purity of medicines, food ingredients, and dietary supplements. To keep that approval, manufacturers are monitored for compliance.

The alleged violations that prompted stores in New York to remove some supplements in February were not questions of content, but or unsupported health claims. The attorney general demanded evidence to back up label claims on Ginkgo biloba, St. John's Wort, ginseng, garlic, echinacea, and saw palmetto pills, among others.

That echoes the concerns of doctors who prefer that patients steer clear of so-called natural supplements.

In October 2013, Children's Hospital of Philadelphia removed supplements from its pharmacy, citing in a statement the lack of "sound information about adverse side effects, drug interactions, or even standard dosing for the vast majority of them."

Sarah Erush, pharmacy clinical manager and a member of the hospital's therapeutic standards committee, wrote that "administering these medications - particularly to children with serious health complications - is unethical when the risks are unknown."

What to Watch Out for In Buying Supplements Online

As problems with nutritional supplements are exposed, some big retailers are getting cautious. But you can find plenty of product online. and that, according to the nonprofit Center for Science in the Public Interest, is a dicey place to shop.

Cautions from its senior nutritionist, David Schardt:

Many testimonials are phony. Initially skeptical, "Amy" now gushes about the secret anti-aging product she found. In fact, she has found lots.

Free samples can be costly. A 14-day free sample costs just $4.99 for shipping and handling. But if you don't cancel within 18 days of the order, you might find a $75 charge on your credit card for the next month's supply, and the next, and the next.

Many "studies" are worthless. That "clinical study" did not compare the supplement with a placebo, so it could not even have found that the supplement worked better than nothing at all.

Read the small print. Such as "Testimonials are based on the experiences of a few people and you are not likely to have similar results."

"News" sites may be fake. Those news organization logos on the page? Well, they're just there. They have no affiliation with the site.

Most "tricks" aren't there to help you. Click on "One Weird Old Tip," and eventually the site will try to get you to buy an overpriced supplement.

SOURCE: CSPI's Nutrition Action Healthletter