(Journal Of The Science Of Food And Agriculture) -- The real amount of active ingredient in over-the-counter St. John's wort "varies greatly" and can be up to 114 percent of the amount on the label, according to new study in the Journal of the Science of Food and Agriculture.

Professor Miao-Lin Hu and fellow researchers at the National Chung-Sing University in Taiwan looked at five commercial St. John's wort products, purchased from ordinary Californian health food stores. They analyzed the amounts of hypericin and pseudohypericin, the two active ingredients that are reported to have antidepressant and antiviral effects. They then compared their results with the manufacturers' label.

None of the labels claimed to contain pseudohypericin, although in actual fact it was present in "much higher" quantities than hypericin.

Actual amounts of hypericin ranged from 1.7 to 38.5 percent of the claimed amount.

When total hypericin (both active ingredients) was measured, the actual figure was up to 114 percent of the claimed amount.

The active ingredients in St. John's wort affects the same metabolic pathway as other prescription drugs used to treat depression and migraine, as well as drugs commonly used to treat heart disease and some cancers. There is also concern that it can affect the contraceptive pill. Most St. John's wort packaging carries a warning to consult a doctor when taking prescription drugs for these and other conditions, but doctors currently have no way of knowing how accurate the rest of the label is.

"Inaccurate labeling has at least one of two effects. The first is potentially to lead to incorrect dose when the label information is complied with. The second is the potential to degrade the perceived significance of the label information amongst either dispensing practitioners, or the patients. This perception could then carry over to more significant drugs," said Jonathan Berman, Health and Safety Group, SCI (Society of Chemical Industry).