Taking too much vitamin D3 can cause calcium levels to reach dangerous levels, and taking a calcium supplement on top of vitamin D3 puts the patient at further risk, states WebMD. Too much calcium in the body leads to nausea, vomiting, loss of appetite, increased thirst and constipation.
Most vitamin D3 supplements are available in 400 IU and 1,000 IU doses, according to Everyday Health, and no person should exceed 4,000 IU of it in a single day. Some manufacturers produce supplements that contain both calcium and vitamin D3. These supplements are generally recommended for people over 50 years old who have osteoporosis.