The popular herbal remedy St. John's wort has led to at least two unwanted pregnancies in Sweden because of its ability to override the effects of the pill, reports Reuters Health.
Sweden's Medical Products Agency website reported that two women, aged 28 and 31, who were both on the pill, became pregnant after taking St. John's wort. The agency also noted that similar cases have been reported in Britain.
Sweden had already introduced a warning label on certain types of St. John's wort products at the end of 1999 after studies showed these products could interact strongly with medical drugs.
An agency official said a similar warning will be placed on all other kinds of St. John's wort products except tea.
"It is still popular but I think the warnings we have put out before have affected sales in Sweden," the Medical Products Agency's Barbro Gerden said.
St. John's wort which is often used to combat mild depression, has prompted several warnings from health authorities in other countries. There have been reports that it may counter the effect of drugs treating HIV infection, blood-thickening drugs and cyclosporine, a medicine used to prevent rejection of organ transplants.
The Swedish authorities explained the effects of the herb by its ability to increase the level of enzymes that break down medical substances.
Gerden also added that despite the labelling on St. John's wort in Sweden, several women on the pill did not consider it to be a drug and therefore did not take precautions when using the herb at the same time.
"Considering the number of women this is affecting, this is important information," she said.
The drugs agency said the herb's effects may last for two weeks after use and recommended that people taking it at the same time as a drug should consult their doctor.
St. John's wort is a plant with yellow flowers that has been used for centuries in many folk and herbal remedies. Its extract is commonly sold in health food stores as a nutritional supplement, promoted as a 'natural' way to improve one's mood.