Vitamin D toxicity has been shown when taking 50,000 international units per day for several months, says Mayo Clinic. For adults, the most that can be taken safely is 4,000 IU per day, reports MedlinePlus.
While rare, vitamin D toxicity, also known as hypervitaminosis D, can cause serious conditions such calcium build up in the blood, says Mayo Clinic. Hypervitaminosis D is almost always the result of over-supplementation, according to MedlinePlus. Those with health issues, such as kidney or liver conditions can be more susceptible to vitamin D toxicity and should take care when supplementing with vitamin D, says Mayo Clinic.