The Medicines and Healthcare products Regulatory Agency (MHRA) has warned people against buying herbal slimming aids from the internet.
The watchdog said there were concerns that some of the aids could contain the prescription-only drug sibutramine.
Many European countries have withdrawn the drug after it was linked to an increased risk of heart attacks or stroke.
The MHRA has warned people to consult their pharmacist, doctor or nurse before using aids they have bought off the internet.
Richard Woodfield, MHRA group manager for herbal medicines policy, warned that people were at risk of purchasing medicines that were not safe or suitable for them.
He warned: “People often assume that because herbal medicines and products contain natural extracts they are automatically safe but this is not the case - no medicine or product is risk free.”