Researchers 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 over 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, including a team from Liverpool John Moores University, bought four samples of the supplement.
Tamoxifen was found in three samples but not in the fourth.
The product label suggested a dose of two capsules a day, which in the case of one sample may have provided 7.6mg of tamoxifen, the researchers said.
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Supplements To Stop Taking (And What To Take Instead)
The medical profession would prescribe 10 to 20mg per day for the clinical treatment of breast enlargement.
In their letter to the British Medical Journal (BMJ), 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"Some of these substances have been withdrawn from use in medicines owing to safety concerns, others have never been tested in humans.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.
"Healthcare professionals should ask their patients about their use of 'supplements' and report suspected adverse reactions."
A spokesman for the Medicines and Healthcare Products Regulatory Agency (MHRA), said: "This research highlights the need for consumers to be aware when choosing their sports supplements.
"These products may claim to increase performance but can 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.
"The MHRA welcomes and will investigate all complaints on sports supplements containing medicinal ingredients. "