There are problems with taking high doses of Vitamin D3 that may be an allergic reaction or genetic, and controversy that the amount of non-supplement D3 consumed by people on average may be a lot higher than we think.

http://www.mgwater.com/vitamin_d.shtml http://www.livestrong.com/article/302715-risks-of-too-much-v...

(result of random refference googling)