I've noticed I can have different physiological responses to the same dose (same batch, same time of day, etc.) I took the stuff regularly for several weeks and mostly stopped because of two side effects - false sense of urgency about everything, and waking up unusually early the day after taking a dose (I was using it for productivity, not sleep avoidance). But I did not notice a drop-off in effectiveness. Still seems to work the same way on the rare occasions I take it now.
