The Medicines and Healthcare products Regulatory Agency, an executive agency of the Department of Health, is advising consumers to exercise caution when considering traditional Chinese medicines to help with weight loss, since some of them may be illegally adulterated or contaminated with pharmaceutical drugs or toxic herbal ingredients.
"We recognise that many consumers value traditional Chinese medicines but they should be aware that we continue to find products manufactured to low quality standards that contain potentially harmful substances," said Roy Alder, director of executive support at the MHRA.
"Many of these products are promoted as natural and safe and hazardous ingredients may not be declared on the label."
In the UK, nearly 66 percent of men and 60 percent of women are either overweight or obese.
While with men, the majority of these are overweight, more than a quarter of women are classified as obese (having a body mass index over 30).
The UK's Chinese herbal medicine industry was worth an estimated £54 million ((€79m) in 1999 although it is difficult to quantify.
According to the agency, a number of dangerous product have been found to be sold on the UK and international markets.
A product called Shubao - Slimming Capsules was last year suspected of having caused a case of irreversible liver failure in the UK.
The Shubao product is believed to be promoted as only containing botanical ingredients but suspected to illegally contain undeclared Nitrosofenfluramine, a drug closely related to the prescription-only fenfluramine that is known to be toxic to the liver.
Other slimming aids contributing to the concern include Qing zhisan tain shou, Li Da Dai Dai Hua, MEIZITANG, Xiao Pang Mei (Pian Shi Yong Shuo Ming Shu) or New Reducing Medicine, Shubaojianfeijiaolang and Jingzhi Kesou Tanchuan.
Some of these products have been found to contain prescription-only medicines like sibutramine and methylphenidate, both of which may raise blood pressure.
The latter may also adversely affect the efficacy of antidepressant and antipsychotic drugs.
A substance banned in the UK called aristolochia that has been linked with kidney failure and cancer, has also been found in herbals.
"The herbal sector supports these actions and is working constructively with the agency toward improved regulation," said a statement from the MHRA.
Under the 2004 European Directive on Traditional Herbal Medicinal Products, EU member states have until 30 October 2005 to implement a registration procedure whereby manufacturers must show they adhere to safety and quality standards, and that they communicate appropriate safety warnings.
Current UK regulations mean that unlicensed herbal medicines do not have to meet set standards of quality and safety.
However, if products are found containing illegal or potent ingredients, such as prescription-only medicines, the MHRA can remove them from sale.
The MHRA has previously expressed fears that the directive could lead to those wishing to get rid of low grade unregulated products and ingredients targeting areas of ineffective regulation to off-load their stocks.
This is not the first time the MHRA has warned about the safety of Chinese medicines sold in the UK.
Last September it issued advice that consumers should avoid products not labelled in English.
External links to companies or organisations mentioned in thisstory: MHRA