I was recently contemplating why supplements seem to be effective and then stop working. Most of the supplements I've used have some sort of affect on the immune system. This effect seems to boost the immune system, but then it seems that a negative feedback occurs and the immune system down regulates itself after a certain period of time. Or, perhaps you boost part of the immune system too much. It could be also be because their is some rate limiting factor such as vitamin or mineral, kind of like methylation. You get the gears turning again, but then you run out of something else.