2) 5-alpha reductase inhibitors (5-ARIs). These include the standard finasteride (sold under names Propecia and Proscar) and the more aggressive dutasteride (sold under name Avodart). They work by reducing the amount of dihydrotestosterone (DHT), the androgen principally responsible for MPB. They are typically administered orally, although topical solutions exist too (albeit at a higher price); the oral solutions have (relatively) little side effects.
I am not sure the "little side effects" part is accurate. It has become common wisdom in some circles, albeit without quantitative proof, that the incidence of side effects is almost certainly larger than the official FDA inserts based on the original studies indicate. Could trials have been biased? Who knows... The percentage of users experiencing side effects such as lowered libido, gyno, loss of nocturnal erections, and changes of body composition over longer periods appears to be quite significant.
For some users the side effects can be very serious. There is a lot of anecdotal evidence that some users can experience long term or even permanent hypogonadism, ED, loss of libido, brain fog, depression or brain fog, gyno, and so on. Reports can be found all over, including at propeciahelp.com, on bodybuilding sites, a couple here on imminst.org, and so on. It is fairly typical to find threads starting with "I used propecia for x amount of time" posted by users desperate to find a way of reestablishing a normal hormonal profile and/or recover normal sexual, physical and mental functioning.
As for dutasteride, hair loss trials were initiated but a decision was made not to apply for FDA approval. It is unknown why but the possibility exists that side effects were too severe.
Edited by andre, 26 May 2009 - 06:18 PM.