Illegal sports supplements could cause kidney failure, seizures and heart problems, medicines experts warned.
Illicit supplements which claim to boost energy or enhance muscle growth could contain dangerous ingredients, the Medicines and Healthcare products Regulatory Agency (MHRA) said.
Two men in Brighton were left severely ill after taking a supplement called Celtic Dragon, an MHRA spokesman said.
One of the men was in such a bad condition that medics thought he would need a liver transplant.
The men, one in his late teens and the other in his early 20s, were admitted to Royal Sussex County Hospital in March suffering from severe jaundice and symptoms of liver dysfunction. They have since fully recovered.
Today, the MHRA launched a campaign warning people not to buy illegal sports supplements after it found 84 products for sale that contain dangerous ingredients such as steroids, stimulants and hormones.
It has also asked major supplement suppliers to submit their products for review, especially supplements containing Ephedrine, Synephrine and Yohimibine which have been linked with side effects such as kidney failure, seizures and heart complications.
The MHRA's borderline medicines section manager, David Carter, said: "People need to be aware that buying illegal sports supplements can seriously damage your health.
"The products may claim to boost your energy or muscle but they could contain unapproved ingredients that can cause kidney failure, heart problems or seizures.
"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."
Andy Parkinson, chief executive of national anti-doping agency UK Anti-Doping, added: "Elite athletes need to exercise extreme caution when it comes to deciding what they put into their body and a vital part of our prevention programme is educating athletes in the risk of supplements.
"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.
"UK Anti-Doping continues to work closely with the MHRA to protect the health of athletes and to prevent doping in sport."