Most supplements are a total mugs game. You have no idea if you are getting the floor sweepings, or some chemical from China.
Obviously there are some reputable ones of course, but there are so many interactions with regular drugs, and other supplements too, NOBODY can ever really be sure of anything.
Even now they are already saying higher levels of VitD3 can actually cause arterial problems, so its turning out possibly not to be the panacea folks thought it was in the first place. Plus you get load of idiots overdosing too.