I think I can tell a marked difference in my day when I have taken a multi-vitamin and when I have not. Not that it is the difference between a great day and a horrible day, but I can tell if I forget. Maybe I have some kind of vitamin deficiency that a multi-vitamin helps to overcome? Who knows. Hell, it could be placebo effect, only like Darkhorse said, I don't expect anything out of it. It is something I noticed rather after the fact. I had been taking multi-vitamins for a while and then I missed a few days and felt kinda run down, etc. Could be coincidence, but like clock work it happens if I stop taking them. I will also say that I can't feel the difference between one brand or another. It isn't that I feel one is superior to another and I tend towards something cheap for the most part. I tried something like Animal Pack and it doesn't do me any more good than GNC or even Rite Aid brand. But without any I can feel it.