The prostate develops and grows in partial response to hormones that are produced in the testes and adrenal glands. Most importantly among these is the hormone testosterone, which is converted to dihydro-testosterone. The enzyme responsible for this conversion is 5-alpha reductase. Normal function of this enzyme permits normal development of male genitalia, including the prostate. Families in which the gene for 5-alpha reductase is mutated have male children with feminized by what is known as 5-alpha reductase deficiency.
In adult life, 5-alpha reductase seems to permit growth of prostatic epithelium: The cells that line the ducts of the prostate and are responsible for forming its secretions. Accordingly, medications that block the action of 5-alpha reductase (finasteride, dutasteride) have been proposed as treatments for over-growth of the prostate, which is exceedingly common as men age. To this end, they have been effective in relieving the symptoms of prostate enlargement. In addition, there is evidence that they can partly reduce the risk of prostate cancer.
So what about using finasteride or dutasteride for chronic prostatitis?
This idea has been examined in clinical trials. Among the trials is a randomized comparison of finasteride and phytotherapy with saw palmetto. In this 2004 study saw palmetto was completely ineffective in relieving the symptoms of type III prostatitis (chronic pelvic pain syndrome). Finasteride, by contrast, was associated with a 5.8-point drop in the CPSI score, from 23.9 before treatment to 18.1 after 12 months. One might note that in other studies, place was associated with a 3.4-point drop and therapeutic massage (not prostate massage) was associated with a 7.6-point drop.