Originally Posted by MinnMan

Before my celiac disease was diagnosed, I developed significant B-12 deficiencies owing to lack of absorption (it's known as pernicious anemia). I was given injections with tremendous amounts of B-12. Boy, it felt great to get over the fatigue of that anemia. And as others have said, you can't overdose on it.

But even people with healthy guts absorb only a fraction of many types of vitamin supplements. I take regular supplements anyway (multivitamin, B-12 and D, which is another for which I previously had deficiencies), just to keep topping up the tank, but it's far better to get these things from a varied and balanced diet.

Now, vitamin A poisoning, that's a real thing