Evidence from clinical trials shows, with a wide margin of confidence, that a prolonged intake of 10,000IU/d of vitamin D3 poses no risk of adverse effects for adults, even if this is added to a rather high physiologic background level of vitamin D.
Vitamin D and cancer mini-symposium: the risk of additional vitamin D.
Vieth R.\nAnn Epidemiol. 2009 Jul;19(7):441-5. Epub 2009 Apr 11.
PMID: 19364661
doi:10.1016/j.annepidem.2009.01.009