A popular sports supplement was banned in the UK this week because it contains unlicensed medicines.
Jack3D uses a substance called DMAA, which is extracted from the geranium plant.
The substance is banned by the World Anti-Doping Agency and is an unlicensed medicinal product, the Medicines and Healthcare products Regulatory Agency said.
The chemical is said to raise the heart rate and narrow the arteries and has been linked to at least one death globally, the MHRA said.
David Carter, of the MHRA, said: "People need to be aware when choosing their sports supplements. These products may claim to increase performance but contain powerful ingredients which can have serious side-effects.
"We recommend that people only use approved products and speak to a qualified medical practitioner if they have any concerns about any supplements they may be taking."
Graham Arthur, of UK Anti-Doping, welcomed the move as a "significant step forward."
He said: "Athletes who use sports supplements need to choose reputable manufacturers who can justify their claims with scientific evidence, and have their products screened to minimise the risk of testing positive for a substance on the World Anti-Doping Agency's Prohibited List."