Researchers found that out of sixteen products that contained St.John's wort three failed to pass independent testing due to cadmium contamination. The three products exceeded the limit established for medicinal plants by the World Health Organization by 100 - 1000 per cent.
One of the three also had lead contamination above the limit established by the State of California.
Two products were dropped from testing because they did not identify the part of the herb used, as required by the FDA. A sixth product failed for suggesting a dose that was likely to be too low to be effective - less than one quarter of the standard dose.
The researchers at Consumerlab.com evaluated St. John's wort products to determine whether they contained the claimed and expected amount and type of St. John's wort. They report their findings on their website.
A benchmark of the quality of St. John's wort extract is the indicated content of hyperforin and hypericin. Purified hyperforin produces the same effect on neurotransmitters as whole St. John's wort. However, products with little or no hyperforin have also shown effectiveness in double-blind studies.
There has been concern over the effectiveness of St. John's wort as the active ingredients are known to affect the same metabolic pathway as other prescription drugs used to treat depression and migraine. Drugs commonly used to treat heart disease and some cancers may be affected too. There is also concern that it could disrupt the contraceptive pill.
The Journal of the American Medical Association reported back in September that long-term use of the herbal St John's wort may reduce the efficacy of at least 50 per cent of drugs taken by Americans, owing to its effect on cytochrome P450 enzymes (CYP) - a group of enzymes that control the concentrations of many endogenous substances and drugs.
Tod Cooperman, President of Consumerlab.com said: "If not properly made, St. John's wort, like other herbs, may be contaminated or lack the right plant chemicals."
"While you can't always tell what's in the bottle, be sure that it claims to be made from flowers or leaves and know whether it is an extract or dried herb. Extracts are two to five times more concentrated than the herb, so the dose needs to be much higher for herb than extract."
As no government agency is responsible for routinely testing St. John's wort supplements for their contents or quality, warnings on St John's wort packaging to consult a doctor when taking prescription drugs essentially provide little advice because doctors currently have no way of knowing how accurate the rest of the label is.