It can be unsafe to take 1,000 milligrams of vitamin D3 daily. Taking vitamin D daily for prolonged lengths of time in amounts that exceed 4,000 IUs (100 micrograms) may cause high calcium blood levels, notes WebMD. The tolerable upper intake level for vitamin D is 4,000 IUs for people aged 9 years and older, as noted by the National Institutes of Health's Office of Dietary Supplements.
The tolerable upper intake level refers to the amount that must not be exceeded in one day. However, doctors may prescribe a short-term treatment of increased doses of vitamin D for certain medical conditions. In regards specifically to vitamin D3, 100 micrograms of vitamin D3 when taken orally for a 3 month period daily may be safe and causes no side effects, as noted by Mayo Clinic.
Vitamin D3 and D2 are two forms of vitamin D. Vitamin D is needed by the body for the absorption of calcium. However, an excess of vitamin D can cause side effects, such as feeling weak or tired, headaches and dry mouth, notes WebMD.