Aside from tolerance, I can think of a couple of possibilities. If the supplement is something your body needs, you feel the obvious difference. After a time you may not feel the difference because your levels have increased to where you're no longer in deficit ("in the red"), so you may discontinue, but upon cessation those levels may slowly decrease until there's a deficit again. You may be close to the balance point, and those "emergency" uses may be enough to bring you up into "the black" again - for a time. I think this is why most people just continue on lower doses than those they may have started on - to keep their levels "in the black". This all depends on what supplements we're talking about, and the underlying need/reason for taking them.

I experienced this with vitamin D. I was very low at one point, brought it up with the megadose type (which I felt at first, but not later) and once the level was up, decreased to something more reasonable. If I stopped taking it altogether, I have no doubt I would become deficient again.

Do you get a sense of anticipation when you start one of these supplements? If so, there's a possibility that adrenalin or one of the other adrenal hormones is involved, making you feel better for a short time, and as time goes on that effect levels off too as levels return to normal.

Doc