Scientists at the University of Stanford have been analysing the drug finasteride, which is administered to men identified as requiring preventive prostate cancer treatment.
They set about trying to discover why, despite the drug preventing cancer in one in four participants, those who did develop it suffered a more aggressive form of the disease.
Their findings, which will be published in the July 7th edition of Clinical Cancer Research, revealed that the drug did not cause aggressive cancer - in fact it made it easier to diagnose.
Dr Joseph Presti, Thomas A Stamey research professor in Urology and director of the urologic oncology programme at Stanford, commented: "We're showing that this is all related to size."
An original cancer trial researcher Dr Catherine Tangen, statistical principal investigator for the Prostate Cancer Prevention Trial and a member at Fred Hutchinson Cancer Research Center in Seattle, agreed that the observations "are consistent with everything we found".
However, she did add that only by removing and analysing the prostate of every man involved in the study would the extent of cancer prevalence by revealed.
Prostate cancer is more common in men over the age of 50 and presents symptoms such as difficulty urinating.