From research around the Internet, D3 is generally seen as safe (
http://www.mayoclinic.org/drugs-supplements/vitamin-d/safety... is pretty typical on the safety information) Personally I take a D3 supplement of 2000IU/day during the dark months (along with some other supplements due to a mostly vegan diet, some of which contain low D2/D3 amounts, so my actual supplemented intake is around 2400 IU).