ummmm do you have baldness in your family? because if you don't you'll be wasting your time on the rogaine and propecia and so on....
if you do then yes, taking them and then just up and stopping will cause you to lose whatever they were helping you keep...
if you're only losing your hair cause you pull it out, then quit pulling it out. i'm a HEAVY perpetrator of that, so i understand that's waaaaay easier said then done, if anything i just trained myself to yank hair out of other places then the top of my head
if you don't have MPB then taking the meds to treat it won't do anything except take your money you could be spending on the more finer pleasures in life such as sorority girls' vodka (which at 19 is a tricky but awesome way to spend money...)
here's hoping you're not like me and realized way too late that you're pulling hair is ill advised... i have two spots near my crown that are more than a LITTLE more vacant than they should be