Doctors can't prescribe anything without actually consulting with a patient, but when the top infectious disease guy tells us he's taking 6000 iu per day, you would have to be a moron to ignore that. I actually take about 8000 iu per day, because a high (but still safe) level is better for MS: Symptom-free for years!
Some doctor in South America used D3 supplements to get MS patients up to 250 ng/ml. They had to very strictly limit their calcium intake so they didn't get organ calcification, but I think all of them had their MS symptoms disappear. Bad stuff can happen at over 150 ng/ml long-term, so this is not recommended, but it's very interesting for MS researchers. I measure my level twice per year: It's under 100 ng/ml, I take no calcium supplements, and I do take Vitamin K2 to keep excess calcium out of my organs.