Saw palmetto fruit contains phytochemicals that inhibit 5-alpha-reductase and aromatoase that cause BPH development.

As men age, the enzyme called 5-alpha-reductase converts the male hormone testosterone to dihydrotestosterone, reducing the level of testosterone in the blood. This signals the enlargement of prostate gland that encirlces the urinary tract to compensate for the deceased level of testosterone level by producing more, constricting the urinay passage. BPH (benign prostatic hypertrophisms, meaning enlargement of prostate) causes poor and frequent urination, typically nightly urination, and sexual dysfunction.

5-alpha-reductase activity is also ascribed to the age-related hair loss.

Both anecdotal evidences and clinical trials indicate that Saw palmetto extracts is indeed an excellent diuretic, i.e. improves urination, reduces urine retention in the bladder, and improves libido and sexual function, without side effects. In contrast, the synthetic BPH drug finasteride showed side effects such as decreased libido and impotence. There have been numerous clinical trials on Saw palmetto extract alone and comparatively with synthetic drugs.