Health savvy people know that most folks should be supplementing vitamin D3. But, what's less known is that if done incorrectly, it can be fatal. That is not one of the sensationalist comments that's so common on online media today. It's accurate clinically, the mechanism of the danger is understood scientifically, and still, it's not well known to public, which has to change.
All of the fat soluble vitamins (A, D, E, and K) are toxic if consumed at high levels, however, if they are supplemented in the correct ratios, it appears that the known forms of toxicity are removed.
The problem with vitamin D is that it improves the absorption of calcium into the body from the intestines, but it doesn't dictate where that calcium should go. Without any other kind of direction, the calcium is absorbed into the blood, and then, because the amount of calcium in the blood is tightly controlled, it can't stay there and gets deposited in varied areas of the body that have a lot of blood flow, like the arteries and kidneys, almost like silt building up on a river bank. This leads to brittle, calcified arteries, prone to stroke, and kidney damage.
The answer is vitamin K2 supplementation. Vitamin K2 activates proteins that direct the calcium into the bone (osteocalcin) and inhibit its deposition into the soft tissues (matrix gla protein). This prevents the risk of cardiovascular disease that otherwise can accompany high dose vitamin D supplementation.
Also, vitamin D requires vitamin A to work well. One: vitamin A improves vitamin D's absorption in the intestines. I have seen patients supplement vitamin D for years without much improvement in their blood levels, but when they switch to a product that contains the right level of A, the blood levels pick up significantly within weeks. Two: vitamin D doesn't just manipulate calcium, it also attaches directly to the DNA in almost 3000 different places on the genome. This attachment requires vitamin A in order to be strong. Without it, it binds weakly, and has a lesser effect on genetic expression. Three: because of the importance of vitamin D's need for A, when it's dosed at high levels without A at the same time, it uses up the A available, causing a vitamin A deficiency, meaning that the other functions that A has, in the immune system and elsewhere, are starving for it.
For these reasons, I almost always give the three together. There are a few products that have these in a single capsule. The one that I use the most is Vitamin D3 Complete from Allergy Research Group.
The material above is not meant to be taken as medical advice, nor is the information here complete enough by itself to make accurate or wise treatment decisions. Please talk with an appropriately trained healthcare provider before supplementing specific nutrients at home.