Proscar (finasteride) and Avodart (dutasteride) are prescriptions pills FDA approved to treat enlarged prostate, or BPH  They are in a class of drugs call "5 Alpha Reductase Inhibitors".  They are not FDA approved to treat prostate cancer (in fact the FDA made a ruling about this in 2012) , or prevent prostate cancer, but they have been shown in some studies have an effect on both prevention, and treatment.  PCRI's Executive Director, Dr Mark Scholz wrote more about this in 2014 - Avodart & Proscar, and in 2011 - Avodart & Proscar for Men on Active Surveillance.   Proscar and Avodart also:

Dr Charles Snuffy Myers also uses this as part of his clinical practice:

See his short videos - 

Avodart for BPH + Active Surveillance

Proscar Vs Avodart

Avodart in Depth

Avodart + Active Surveillance

Avodart + PSA-Only Recurrent PCa

Proscar/Avodart + High Grade PCa

Weigh the risks and benefits for yourself.  Discuss with your physician(s).