Modafinil
"My first 30 days on Provigil (200mg x 2 daily) were literally a life changing event.  I have never felt so energetic, awake, and constantly in a good mood in my life.  However, after 30 days the effects started to wane and, after 90 days, I feel little benefit.  I tried taking an additional 3rd supplemental dose of 100mg a couple of hours after my second daily dose of 200mg to see if the good results might return, but all I gained from the additional 100mg was a headache.  There is a definite and very unfortunate tolerance problem with Provigil in my experience."