According to US study about Chinese herbal medicine.

A toxic ingredient in a popular Chinese herbal remedy is linked to more than half of all cases of urinary tract cancer in Taiwan where use of traditional chinese medicine is widespread.

The latest research found Aristolochic acid (AA) can interact with a person's DNA and form unique biomarkers of exposure, as well as creating signals within tumor suppressing genes that indicate the carcinogen has been ingested.

The Chinese herb is known in Europe by the name birthwort because it was often given to women during childbirth. Derived from the Greek, "aristolochia" means noble birth.

Natural is not necessarily safe.....Doctor says.Once again, as is often the case, the herbs used in Taiwan come from China. We've read time and time again about herbal remedies from China containing mercury and all sorts of toxic metals. You should take care of this....

Refrence:Yahoo health news 10th,April,2012