I use 5,000 iu/day without an issue, with a little magnesium.
When your blood levels of vitamin D3 won't change, some CoQ10 or Ubiquinol will help the intestines absorb it- it's a little-known cofactor.
Also, always take your D3 together with a meal with some sort of fats. If your stomach goes runny when ingesting fats, you need a little niacin with your meals (less than 100 mgs, non-deposit tablets).