My family is in this group. We are poor absorbers of vitamin D, some of my elder relatives need 5 times the "safe upper limit" to have healthy blood levels. As long as you're checking your blood values routinely (and for both D2 and D3, not just one or the other), it's reasonably safe. Sort of like other prescriptions in general.
I heard about a guy who ordered a bottle and ended up with vitamin D poisoning, on one of those Ira Glass style podcasts. Turns out they forgot to compound it before sending it out so he was getting “cask strength” vitamin D. Sounded very unpleasant.
