The medicines watchdog has issued a warning about illegal and potentially dangerous traditional Chinese medicine slimming aids available in the UK.
People trying to lose weight may try Chinese slimming aids
The Medicines and Healthcare products Regulatory Agency (MHRA) says some may contain potentially harmful substances.
A drug which raises blood pressure and an amphetamine-like stimulant have both been found in slimming aids.
The MHRA said the herbal medicine sector backed its efforts to ensure only safe products were available.
'Promoted as safe'
Some Chinese medicine slimming aids have been found to contain sibutramine, a prescription only medicine that should only be used under medical supervision as it may raise blood pressure.
Others contain an amphetamine-like stimulant called methylphenidate that may lead to high blood pressure and adversely affect the efficacy of antidepressant and antipsychotic drugs.
Aristolochia, a banned substance often found in Chinese medical products and linked with kidney failure and cancer, has also recently been found in UK herbal medicines.
The MHRA said one weight loss remedy - Shubao Slimming Capsules - was thought to be responsible for a UK case of irreversible liver failure. The patient in that case required a liver transplant.
Roy Alder, Director of Executive Support at the MHRA said: "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.
"Many of these products are promoted as natural and safe and hazardous ingredients may not be declared on the label."
The MHRA said it aimed to reduce the availability of dangerous herbal slimming aids through laws, consumer advice and improving the regulation of unlicensed herbal remedies.
A spokesman added: "The herbal sector supports these actions and is working constructively with the agency toward improved regulation."