I take fish oil and it helps me think more clearly. I took a B complex for a while and it really helped my energy, since I was probably deficient. I bet it wouldn't do anything if you already had enough B vitamins. Vitamin D I take sometimes because I avoid being out in the sun, and I occasionally (but not regularly) take a multivitamin since I don't always eat so well and I don't want to become deficient in anything. I think they all have helped me, but some multivitamins are better than others - some create expensive urine as said above but others don't. I cut mine in half since they put so much of everything in there.