The lower incidence of prostate cancer is postulated to be related to diabetes, not medications.

Users of antidiabetic medications have a lower risk of prostate cancer than non-users, according to researchers in Finland.

The investigators, led by Teemu J. Murtola, MD, of the University of Tampere, compared 24,723 prostate cancer patients with the same number of controls individually matched by age and residential area at the time of the corresponding case’s cancer diagnosis. Oral antidiabetic drugs were used by 7.5% of prostate cancer patients compared with 8.4% of controls.

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The use of any antidiabetic drug was associated with a 16% decreased risk of prostate cancer, Dr. Murtola and colleagues reported in the American Journal of Epidemiology (2008; published online ahead of print). Overall risk decreased with the amount of oral drugs and insulin used.

Additionally, duration of antidiabetic drug use was inversely related to cancer risk. For example, those who had been on antidiabetic drugs for one year or less had only a 4% reduced of prostate cancer overall and of advanced cancer compared with men not using antidiabetic medications. Individuals taking the drugs for seven years had a 34% and 39% reduced risk, respectively.

The observation that prostate cancer risk decreased in users of multiple oral antidiabetic drugs as well as insulin suggests that the risk-lowering effect is associated with diabetes and not antidiabetic drug therapy per se, the authors said.

With respect to study limitations, the researchers noted that they were missing information about PSA testing. Previous research suggests that PSA levels are lower in diabetic than in non-diabetic men, independent of antidiabetic medication use, Dr. Murtola’s group said.

It is possible that fewer prostate cancers are being diagnosed in diabetic men because fewer prostate biopsies are being performed based on elevated PSA levels. The authors pointed out, however, that Finland has no mass PSA screening and opportunistic screening is uncommon, which would decrease the likelihood of such bias.

As for the relationship between diabetes and lower prostate cancer risk, the researchers noted that diabetic men have a lower serum testosterone level compared with non-diabetic men, and low serum testosterone is associated with an elevated risk of diabetes. “Because growth of prostate cancer is androgen-dependent, changes in serum testosterone in diabetic men could provide a possible explanation for their lowered prostate cancer risk.”