Hi,my father vitamin d3 level is 40 mol/l,he has been prescribed vitamin d3 4000iu daily, I was wondering if this can be safely increased, and how long does it take the level to reach 100. We live in United Kingdom, now in autumn,
Generally, the safe upper limit for daily vitamin D consumption is 10,000 IU/day. That is, you can take up to 10,000 IU/day but do not take more than that.
It is hard to say when an individual will reach a specific level. When beginning to supplement or when changing dose it is best to try that dose for about a month and then have your levels measured again. At this time, you will have a better indicator of how that dose is affecting your levels.