Many people think that when they buy supplements in a drugstore, the products are as well tested as pharmaceuticals and contain the compounds they claim to.

But a recent study shows once again that substitutions and quality problems are common. And experts point out that many supplements do not help even at the correct doses.

Canadian researchers tested 44 herbal products from 12 companies, and found that over half (59 percent) contained plants not listed on the labels.

One-third contained contaminants and/or fillers, some of which could pose health risks. And only two of the 12 companies used all-authentic ingredients.

Using a new test called DNA bar coding, which identifies plant species based on short gene sequences, Steven Newmaster of the University of Guelph in Ontario found that many pills contained substitute ingredients and cheap fillers like soybeans, rice, and wheat.

In the study, two bottles of St. John's wort, touted as a treatment for mild depression, had no trace of the herb. One capsule substituted senna, an FDA-approved nonprescription herbal laxative that can cause diarrhea and liver damage and should not be used longer than two weeks.

Several products were contaminated with feverfew, an invasive weed that can cause "swelling and numbness of the mouth, oral ulcers, nausea, vomiting and diarrhea." It also may raise the risk of bleeding if taken with blood thinning drugs such as warfarin or aspirin.

Another Ginkgo biloba product, which is touted to improve cognitive functioning, was contaminated with black walnut leaves, which may have been harvested along with the target crops. The paper notes that walnut leaves contain juglone, which is toxic and a tumor-causing agent.

"For new consumers, it's almost impossible to know what is real or not," says nutritionist David Schardt at the Center for Science in the Public Interest, an advocacy group. "While the DNA biomarker study showed that only half of the herbal products had what they were supposed to have, some had nothing, thereby producing no effect. In general, it's less of a safety issue and more an issue that maybe you're not getting what you think you are.

"What we tell consumers is that if you're taking a botanical product, and it's helping you and it's not making you sick and you're not paying too much for it, maybe you have a good product or a placebo effect. But if you're taking something and it's causing problems, stop."

The American Botanical Council, a nonprofit that promotes the study and use of supplements, took the Canadian paper to task. Calling the new DNA technology "reliable when used appropriately," the council's executive director, Mark Blumenthal, said that DNA-based analysis is not appropriate when used to map herbal products that may contain plant extracts.

Herbal extracts do not necessarily contain genetic material. They are intended to contain just key plant chemicals in higher concentrations than found in the whole herb.

But Newmaster said all the tested items were whole herb products.

Other studies have also documented the unreliability of the ingredients in herbal supplements.

One study showed that in examining 40 varieties of the supplement black cohosh, 25 percent had a substitute ingredient. In another analysis, half of ginseng products labeled "Korean ginseng" were actually American ginseng.

"Many times supplements contain the herb as advertised, but they contain less of it than they claim," said Tod Cooperman, president of ConsumerLab.com, an independent testing laboratory. He noted that this is something "you won't discover in bar coding, which doesn't look at the amount of the ingredient but only if something is or is not there. In any supplement it's important that you're getting the right dose.

"Forty-three percent of herbals we've tested most recently have failed our tests, using analytical chemistry to look at compounds and amounts," noted Cooperman.

Even if an herbal supplement is the right product and the right dose, there is still concern about whether it will work. It depends on the herb, said Cooperman, a physician who gives mixed reviews to echinacea for colds, Ginkgo biloba for cognitive decline, valerian for sleep, and ginseng for energy.

The herbal supplement industry is regulated, but the rules tend to be lax. Over the last few years, inspectors have required companies to practice good manufacturing processes, but many firms are failing inspections.

"They may make products in the same way lot to lot," said Schardt, "but that doesn't mean they're making good products.

"It's really a crapshoot when you buy a product; you have a 50-50 chance of getting what the label says," he said.

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