Bodybuilders may be unaware they are taking supplements containing the breast cancer drug tamoxifen, experts have warned.
Researchers, including a team from Britain's Liverpool John Moores University, said "obscure" references to the drug had been found in one supplement called Esto Suppress.
People trying to increase their muscle mass have been taking tamoxifen to stop breast enlargement caused by the use of anabolic steroids for more than 30 years, they said.
Tamoxifen is usually bought from illegal sources but the experts said there had been speculation on bodybuilding discussion forums that the dietary supplement Esto Suppress contains the drug.
The researchers found tamoxifen in three out of four samples of the supplement.
The product label suggested a dose of two capsules a day, which in the case of one sample may have provided 7.6 milligrams of tamoxifen, the researchers said.
The medical profession would prescribe 10 to 20 milligrams a day for the clinical treatment of breast enlargement.
In their letter to the British Medical Journal, the team said it was not known whether the Esto Suppress currently on sale contained tamoxifen as their analysis was carried out two years ago.
But they warned: "Since the 2000s, a growing number of off-the-shelf 'food', 'herbal' or 'dietary supplements' - aimed at gym-goers and people wanting to lose weight or enhance their sex lives - have contained pharmacologically active substances (including anabolic steroids, stimulants and appetite suppressants).
"Some of these substances have been withdrawn from use in medicines owing to safety concerns, others have never been tested in humans.
"Often the substances are not listed on the labelling, and products may be marketed as 'natural' , exploiting the belief that they are safer and healthier options.
"In other cases, such as with Esto Suppress, only an obscure reference is made to the substance, such as a chemical name.
"Most users will be unaware that they are taking these substances."
The researchers advised healthcare professionals to ask their patients about their use of supplements and report suspected adverse reactions.
