There was some early concern based on the Prostate Cancer Prevention Trial and REDUCE trial that it might cause some increase in high risk cancers, but that was later found to be a result of detection bias (it's easier to find cancer in a smaller prostate) and certain problems in the study design.
Here's what a later study published in the NEJM found:
NEJM said...
The reduction in prostate volume with dutasteride, along with the increase in prostate volume with placebo, could have caused an increase in the number of biopsy-detected prostate cancers among men in the dutasteride group, but the actual result was a 23% relative reduction in prostate cancer, a finding that supports a mechanism of tumor shrinkage with dutasteride. Other biases may enhance the detection of prostate cancer among men who are being treated with 5α-reductase inhibitors, including an improvement in the sensitivity of PSA tests and of digital rectal examination. By requiring a baseline biopsy and biopsies after 2 and 4 years, the design of the REDUCE trial minimized the proportion of study participants who underwent protocol-independent biopsies (≤7%) and minimized biases that could have caused between-group differences in the rate of protocol-independent biopsies.
Biases could explain the relative risk of 1.27 (95% CI, 1.07 to 1.50) for tumors with Gleason scores of 7 to 10 in the finasteride group of the Prostate Cancer Prevention Trial and models that account for volume and PSA-driven biases have suggested that there may be reductions of 12 to 27% in the incidence of tumors with Gleason scores of 7 to 10 with finasteride. In our trial, there was no significant increase in the dutasteride group, as compared with the placebo group, in the incidence of tumors with Gleason scores of 7 to 10 over the 4 years of the trial, both before and after adjustment for possible confounding variables.
www.nejm.org/doi/full/10.1056/NEJMoa0908127Other studies have echoed these findings:
www.mayoclinicproceedings.org/article/S0025-6196(16)30456-6/fulltextjamanetwork.com/journals/jamaoncology/fullarticle/2212207www.nejm.org/doi/10.1056/NEJMoa1215932jamanetwork.com/journals/jamaoncology/fullarticle/2212207www.jurology.com/article/S0022-5347(12)04995-6/fulltextNot only do they prevent low grade tumors, they also seem to shrink tumors that are already there:
www.jurology.com/article/S0022-5347(16)31794-3/fulltextThere may be some adverse sexual side effects and they will re-grow hair. Cialis may improve urine flow and will help with the sexual side effects.