I vary from brand to brand with vitamins, but this is one of the ones that is always in my medicine cabinet. I think if you have a vitamin deficiency or are dangerously close to having a deficiency, you should see your doctor to get Vitamin D2, which is a prescribed and stronger version of D3 that will definitely give you a bigger boost, more energy, and help raise the vitamin D count. My mother was one number away from having a deficiency and these didn't seem to help her enough; she had a lot of muscle pains and aches from having such a low count, so her doctor prescribed her D2 and it greatly helped her (no more aches and pains!). But, for everyday use, just to kind of keep up your Vitamin D count, this is great. The tablets are very small and have a small dosage each, so I usually take two a day. I have a sensitive stomach and can't take things like Centrum, but this is one of the few that doesn't affect my stomach. Don't know how effective this is in improving immune health, seeing as how I take these every day and continue to get sick. But these are okay.