In a letter to the British Medical Journal (BMJ) published this week, researchers reveal the test findings from 2011 and 2012.
Led by Michael Evans-Brown from the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) in Portugal, and in collaboration with researchers from the University of Liverpool in the UK, the testing team found tamoxifen present, and at different concentrations, in three out of four samples.
The team warned that most users, "will be unaware that they are taking these substances" and urged healthcare professionals to ask their patients about their use of such supplements and report suspected adverse reactions.
"The product label suggested a dosage of two capsules a day, which in the case of sample 1 may have provided 7.6 mg of tamoxifen," explained the researchers - who added that 10-20 mg is used clinically for treating gynaecomastia (breast swelling).
Evans-Brown and colleagues noted that it is not known whether the Esto Suppress supplement currently being sold still contains tamoxifen - but added that for more than three decades, bodybuilders have taken tamoxifen to prevent and treat gynaecomastia (breast swelling) caused by use of anabolic steroids.
Usually, they added, tamoxifen is sourced from the illicit narcotic market. However, bodybuilding discussion forums have speculated that the Esto Suppress supplement manufactured by Delaware-based Pharma Labs Inc contained the drug because the label listed one of its less well known chemical names.
In the study the team purchased and tested four samples of the food supplement at different times between late 2011 and early 2012.
The analysis was performed using reference standards and gas chromatography coupled with flame ionisation and mass spectrometry detectors.
"Tamoxifen was found in samples 1 (3.8 mg), 2 (0.9 mg), and 3 (3.0 mg), but not in sample 4," the scientists revealed.
Pharma Labs Inc could not be contacted before publication.
Source: British Medical Journal
Published online ahead of print, doi: 10.1136/bmj.g1476
"Is the breast cancer drug tamoxifen being sold as a bodybuilding dietary supplement?"
Authors: Michael Evans-Brown, Andreas Kimergård, Jim McVeigh, Martin Chandler, Simon D Brand