What is interesting about the 5AR inhibition caused by saw palmetto is that it is prostate-specific. Not only does saw palmetto inhibit both types of 5AR in the prostate but not in other tested tissues (epididymis, testes, kidney, skin, and breast) in vitro [20], the active ingredients of saw palmetto concentrate in the prostate as opposed to other tissues after oral administration to rats [21]. This is in line with human studies indicating that saw palmetto extract preferentially reduces prostatic DHT and does not alter plasma DHT or sex hormone levels [9, 22-23]. This explains the lack of side effects in clinical trials. It also means that saw palmetto is unlikely to prevent hair loss.
It should be noted that even SP's ability to help enlarged prostate has come into question -- interest in the drug gets its main support from a study in JAMA a few years back which found a benefit (big study, big league publication), but a paper published this year -- this time in the New England Journal of Medicine -- found no benefit, and the authors of that study supposedly questioned the methodology of the JAMA paper.