Morning Starbuck,
I realy think this depends on the person, their physiology, and the med. For instance, when I was going thru my trial trying to find the right med(s) for my bipolar, I tried several that worked for a short time, and then pooped out (a technical industry term). One that comes to mind was Wellbutrin. It made me feel great. We worked me up to my target dose (300 mgs I think), it worked well for about 2 weeks, and suddenly, nothing. I had other meds do the same, but I'm not sure which ones. I think Prozac was one. Then I found Zyprexa and Topamax, and they haven't failed me in three years. I haven't got a clue why. My doc is switching me from the Zy to Abilify, and I'm very worried about that. But he hasn't let me down yet, so I'll give it a shot.
This probably didn't help much, but I wanted to give you some feedback.
Greg