You can get some answers on this article:
Saw Palmetto for Hair LossIn theory alone, Saw Palmetto act like finasteride by helping those suffering from BPH (The theorized reason for its effectivity is that it suppresses the enzyme called 5-alpha reductase which in turn prevents the production of the potent dihydrotestosterone (DHT) from the hormone testosterone).
So in theory it might work as a treatment for hair loss but there are no studies to date that support this claim. Also, because there has been no formal studies, dosage and how it should be used (orally or topically) is also a guess. If you check the article above you can see Dr. Cole recommendations on how to use and the dosage.