Vitamin D is not water soluble, so it's important not to get too much.
If you get too much, it can cause toxic levels of calcium in your blood. This weakens your bones but it also fucks up your heart, makes you have to pee a lot, makes you very thirsty, and causes confusion. Vitamin D also directly causes intoxication.
It is possible that my dad's recent decline was entirely triggered by his fucking YouTube health advice.
He'd stopped taking D over a week ago and all of the calcium effects were still getting worse because he was eating more.
Anyway, his doctor thinks this is treatable.