Critics warn that some traditional Chinese medicines contain toxins. (bigstock)

Traditional Chinese medicine is enjoying increasing popularity all over the world. But two recently published studies show that the treatments can be harmful. The papers focus attention on the fact that not all of the ingredients in TCM treatments are listed, or even legal, and that some can cause cancer.

Critics have long warned that some mixtures can also contain naturally occurring toxins; contaminants such as heavy metals; added substances such as steroids, which can make them appear more effective; and traces of animals that are endangered and trade-restricted.

Now, researchers in Australia have investigated the issue using modern sequencing technology. The team analyzed 15 TCM samples seized by Australian officials.

“We took these traditional preparations, smashed them to pieces and extracted the DNA from the powder,” explained molecular geneticist Michael Bunce.

Some products contained material from animals classified as vulnerable or critically endangered, such as the Asiatic black bear and the Saiga antelope, just as the producers of the products claimed. But often, the medicine also harbored ingredients not mentioned on the packaging, the team reported online in PLoS Genetics.

In the herbal preparations that they tested, Bunce and his colleagues found members of 68 plant families, among them plants of the genera Ephedra and Asarum. Both can contain toxic substances such as aristolochic acid, banned in many countries because it causes kidney disease and upper urinary tract cancer, or UUC. Although detecting DNA from a certain species does not mean that a toxin produced by that plant is present, chemical analysis of one of the four samples containing Asarum did turn up aristolochic acid.

The threat posed by aristolochic acid is also highlighted in a paper published this month in the Proceedings of the National Academy of Sciences.

For this paper, researchers focused on Taiwan, which has the highest rate of UUC in the world. A previous analysis had shown that roughly one-third of the Taiwanese population consumed herbs likely to contain aristolochic acid.

The scientists sequenced the tumors of 151 patients with UUC. Among those with mutations in a tumor-suppressor gene that make people more vulnerable to cancer, 84 percent showed indications of exposure to aristolochic acid. The study provides compelling evidence that aristolochic acid is a primary cause of UUC in Taiwan, the authors argue.

Kai Kupferschmidt of ScienceNOW, the daily online news service of the journal Science