The study also said the acids may cause liver cancer, but the CFDA said it saw no direct link to liver cancer in China. Surveys show that most liver cancer cases in China are caused by hepatitis B, it said.

The WHO has included aristolochic acids in a list of cancer-causing substances.

TCM drugs containing aristolochic acids have been used in China for more than 1,000 years. Currently 47 oral TCM preparations containing the substance are available in the domestic market, the CFDA said.

Herb products containing the chemicals have not been approved by the United States Food and Drug Administration and are often marketed as dietary supplements or "traditional medicines", according to the National Cancer Institute in the US.

Some other countries, such as Britain and Germany also have rules that limit or ban the use of the acids, the CFDA said.

Since 2003, China has taken a number of steps to control risks associated with aristolochic acids, including forbidding the use of certain herbs that contain high levels.

With those measures in place, the number of cases of kidney damage related to the acids in China decreased greatly, and no report of kidney cancer directly caused by the acids has been reported, the administration said.

Liang Aihua, a TCM researcher at the Chinese Academy of Chinese Medical Sciences, said drugs are poison in proportion to the amount ingested, as well as their time in the body. International standards have been set limiting the use of other potentially poisonous substances to ensure their safety, she said.

Moreover, TCM has stressed processing techniques to reduce drugs' poisonous nature, and many unique methods have been adopted for safety, she added.

"Long-term use of any drug in large doses will likely result in negative reactions or poisonous effects," she said. "Specifications for the use of TCM drugs should be further regulated to provide alerts to users about possible side effects."