Even 1000IU is a low dose for MS folk. It appears that “we” tend to not only have low Vitamin D levels but our ability to absorb Vit D is impaired as well, so need to take much higher dosage for the right amount to actually get into our systems.
I take 3000IU daily in summer and double up when the clocks go back for winter. I have a softgel tablet and an oral spray and alternate them.