The argument is generally: Vitamin D has a lot of complex interactions in the body and is fat-soluble. That makes it very easy to overdose, and the recommended doses are substantially higher than normal dietary intake. Most studies on vitamin D don't really differentiate between vitamin D from supplementation and vitamin D from going outside, meaning that the protective effect might be from the activity not the vitamin.

If everyone takes 5K IU/day, then you'd expect issues from about half of people (if baseline of 50% of people are low is correct). Those issues are nontrivial.

I personally think that most people (especially those who spend a lot of time indoors) should probably supplement, and most of the reaction is overstated.

50% deficient doesn't mean 50% have too much. It's not a hair-thin line you are balancing. There is a wide band of acceptable values.

One important negative effect of Vit D supplementation is that it reduces magnesium absorption (due to increasing calcium absorption, which is a magnesium antagonist).

It’s no big deal if you have sufficient magnesium, but for people who have low magnesium (which is common) it can make the problem worse.

This may explain the fatigue/lethargy at least one commenter here reported after taking fairly high-dose Vit. D supplements.

IMO this doesn’t mean you shouldn’t take them, but they should be taken with caution and ideally with clinical nutritional profile testing so magnesium or any other deficiencies can be addressed.

I am also have this troubles even from 500 IU (form or vendor of pills doesn't matter) after that i have massive headache with slow pulse in head and unable to concentrate.