The FDA adjusted their guidelines in 2015 in response to this (https://www.chicagotribune.com/business/ct-confusing-drug-ad...).
There was another study that showed that taking 5000 IU or 10000 IU would result in levels well below 210 ng/ml in average people, which is around the level where we'd expect side effects. This is not medical advice. I am not a doctor, just a random guy on the Internet. However, I take 5000 IU daily, and I intend to raise this to 10000 IU when I get vaccine doses. Again, this is not medical advice. Look up the studies yourself, and do your own research.
EDIT: negatively correlated
https://retractionwatch.com/2021/02/19/widely-shared-vitamin...
https://www.medrxiv.org/content/10.1101/2020.09.04.20188268v...
(Also, thanks. I didn't realize the one you cited as retracted, though there's some nuance there too.)
I am not a doctor. You are not a doctor. 'There was another study' is a terrible way to self-medicate. Consequently, 'look up the studies yourself' is bad advice for self-medication.
If you do not have a deficiency, you can severely damage your health by taking high doses, like destroying your kidneys. I guess that happens because those kidneys need to wash out all the crap you feed your body that it doesn't want nor need.
5000 IU is not a low dose and should only be taken if you have a diagnosed vitamin D deficiency.
Also, I've heard of friends and colleagues who felt pretty rough for a couple of days.
But one thing I don't hear enough of, is the comparison with all the long lists of potential side-effects stated on normal, over-the-counter medicines. Paracetamol, Aspirin etc.
Have you listened to the fast-talking voices at the end of medicine adverts, disclaiming a litany of possible side-effects? It's quite amazing. (Some disclaimers at the end of pharmaceutical ads on US TV particularly, have always shocked me.)
"Take this. It'll help your problem" (* warning: may cause ... )