Propecia is the best reasonably priced treatment option for MPB (male pattern baldness) at the moment. It can have side effects though. I believe the clinical trials showed that side effects occurred in around 2% of men. As has been said in previous posts, this drug works by inhibiting your bodies production of an enzyme called 5 alpha-reduclase, which basically binds with free testosterone and converts it into DHT (dihydrotestosterone). The hormone dihydrotestosterone is responsible for the development of male characteristics in males during adolescence - facial hair etc.
Tieing in with what has been said previously about MPB only being a problem in men after the onset of adolescence (after testosterone production has been cranked up), dihydrotestosterone is also thought to be the hormone responsible for causing hair follices to shrink. Therefore, if you can reduce the amount of DHT in your bloodstream, by blocking the enzyme that converts free testosterone to DHT, the theory is that you will reduce the rate of MPB. The clinical trials of Propecia appear to support this theory, with apparently most men tested seeing a reduction in hair loss and a smaller number of men even seeing regrowth.
I'm 26 and have been losing hair for a while. I tried Propecia for a few years and it didn't eliminate my hair loss, but it did appear to significantly slow it down. I made a decision about 6 months ago to come off Propecia however, because I was suffering from side effects including reduced libido and also more worryingly, cognition problems. Having come off the drug, I am no longer experiencing the side effects. Looking around on the internet however, there are calls from a number of doctors for further research into Propecia, because a number of people have reported that they experienced side effects that did not go away after they had come off the drug. This is obviously very worrying for people.
There is research taking place into something called stem cell multiplication of hair follicles. A company called Intercytex based in Manchester, England were conducting clinical trials of a procedure which basically involved extracting hair follicle stem cells from the area of DHT resistant hair follices around the back and side of your head, then multiplying these follices in the laboratory and then injecting these follices into the balding areas on the top of your head. This seemed like a very good idea and if they could get it to work properly and bring it home at a reasonable price, it would be a massive breakthrough in the treatment of MPB. However, at some point last year, I believe intercytex sold this product to an American company. I'm not sure how the research has been going since then, but hopefully, fingers crossed, this will soon be a feasible option for people.