This has been posted here several times. Keep in mind, "raises risk of" does not mean "causes". 5ARI's decrease the overall size of the prostate, which can make tumors more easily detectible during cancer screenings. Because of this, patients who take 5ARI’s appear to have an increased risk of prostate cancer, but in reality tumors are just being found more commonly and easily.
The PLESS study showed an overall ~25% decrease in the number of low to medium grade tumors in 5ARI patients, but a slight increase in high grade (more deadly) prostate tumors. However, I don’t believe the data was strong enough to conclude 5ARI’s were directly responsible for this increase in high grade tumors, but it’s quite clear they can help reduce low to medium grade.
Here is a really great radio interview about this with the chief medical officer of the American Cancer Society:
