When I take vitamin d, in any dose or brand, I often feel not only headachey but very giddy, almost like im high, less anxious too, why?

D giddy. When deficient it can cause problems with memory, fatigue, joint and back pain, demineralization of bone, and likely depression. The relief from any of them would likely promote acute feelings of well-being. Smaller sizes of the tablets (400 iu) allow for taking quarter or half sizes - much less than the typically provided 1000 iu capsules/fat solubles. Blood testing screen. 1500-2000iu suggested.