Does it work?

Saw palmetto has been widely tested, and the results show promise in the treatment of urinary symptoms caused by prostate problems. However, it is important to know that saw palmetto is recommended to treat symptoms associated with benign prostate gland enlargement, not prostate cancer.

Dangers and possible side effects....

it may suppress PSA levels in your blood. This action can interfere with the effectiveness of the PSA test. That's why if you take saw palmetto or other herbal medicines, it's important to tell your doctor before having a PSA test. Avoid if allergic or hypersensitive to saw palmetto. Use cautiously with a history of health conditions involving the stomach, liver, heart, or lungs; hormone-sensitive conditions; or bleeding disorders. Use cautiously with drugs that thin the blood or hormonal drugs.

References:

http://www.webmd.com/vitamins-supplements/ingredientmono-971-saw%20palmetto.aspx?activeingredientid=971&activeingredientname=saw%20palmetto

http://www.healthline.com/health/testosterone-saw-palmetto