The fat soluble vitamins – A, D, E, and K – are the most dangerous in high doses because they are stored in the body. Two of these, A and D, have very clear toxicity. Large doses of vitamin K will cause clotting problems. With vitamin E, strangely enough, we have not seen any toxicity.
This is not to say that water soluble vitamins are safe at any dose. To be excreted in the urine, they must pass through the blood, and in that short time, they may have toxic effect. Vitamin B3 in very large doses causes palpitations, sweating and circulatory problems. And there’s evidence that excess of some vitamins, such as B6, may cause brain damage.