Fred, That warning really has to do with people on dialysis. I have been taking 12,000 IU Vitamin D3 and lots of Magnesium for a year now and have had no problems at all. If you look at hypermagnesemia on Wikipedia you will see that it causes hypocalcemia. Therefore if you are checking your serum Vitamin D levels AND your serum calcium levels regularly like you should be if you are taking more than 2000IU of Vitamin D3/day, you will be at no risk of hypermagnesemia. If the drugs.com warning is to be taken seriously, how do you explain that many multivitamins contain both Vitamin D3 AND magnesium? The answer has to do with the fact that the problem is NOT with how much you are taking, but rather with your blood levels of Vitamin D. You get them too high (200ng/ml+) and it can screw up a lot of things, magnesium being only one of them. That is the part that drugs.com does not explain. They are only covering themselves in light of the fact that there is a certain hysteria in the medical community about imagined risks with Vitamin D. The real message here is that IF YOU ARE TAKING VITAMIN D, GET YOUR BLOOD LEVELS OF VITAMIN D CHECKED REGULARLY until you know how your body is handling it. UNLESS you have another underlying problem, you will always be safe with Vitamin D3 blood levels around 50ng/ml. I would also NOT take more than around 600mg/day magnesium, since it puts additional stress on the kidneys to eliminate it. IF magnesium is giving you the "runs", that is a sign you need to cut back on it. It is also one of the reasons it is hard to overdose on magnesium. It tends to let you know when you're getting too much.