The vast majority of hair loss is caused by genetic and hormonal factors.  However, this does not mean there is nothing you can do to prevent it from happening.  Propecia and Rogaine are the most common treatments for thinning hair, and the only ones approved by the FDA.  For the majority of people, they will slow or prevent further hair loss.  For some, these medications will even cause new hair growth.  However, these medications are far from perfect.  Both medications will stop working soon after you stop taking them.  If you have been using them for 5 years, and suddenly stop, you could lose all the hair that you would have lost during that period anyways.