I understand that people respond very differently to high doses of vitamin D and their baseline levels are not always tested so its hard to evaluate whether additional intake is either needed or safe.
I also understand that Vitamin D toxicity can have adverse health effects, which may not show up until months or even years after starting to take high doses, so it’s not recommended to exceed 4000 IU (100 micrograms) per day.
Larger doses have not been evidenced to deliver any additional health benefits, the research still seems to be inconclusive, and may therefore be completely unnecessary.
My GP checked my levels, prescribed additional Vitamin D to take over the winter, but then when levels came up to upper end of normal range his advice was to discontinue. Which I have. I plan to get my levels checked again as we go into next winter.