Many years ago my young son ate a large number of children's chewable vitamins. Because they contained vitamin D, and the theory among all doctors was that even a slight increase in the consumption of fat-soluble vitamins above the RDA could be toxic, my son was given ipecac by the doctor, which made him ill in a way that was very unpleasant for him.
On another occasion, a similar thing happened with our dog. Only this time when the dog was given ipecac, he aspirated some of the vomit, got pneumonia, and had to be treated in an animal hospital at incredible expense.
In neither of these cases was there 20000 IU of vitamin D in the entire container of vitamins.
So all this induced vomiting and expense was entirely unnecessary.
Doctors and nutritionists have told us for decades that doses of vitamins above the RDA are unnecessary and even harmful. But they have gradually had to retreat from this position, which they have maintained at great harm to millions of people.
Well, vitamin A in large doses can be be poisonous. That is a medical fact. Polar bears store so much vitamin A in their livers that a human who eats it can die.
That is still true if you're talking fat-soluable vitamins such as A and D--high doses are actually poisonous. But water-soluable vitamins such as the B series, C, and a few others are relatively safe even at 500-700% RDA on a daily basis.