The maximum safe dosage of vitamin D-3 per day for adults is 4,000 international units, reports MedlinePlus. This would equate to 16 capsules at 250 international units of vitamin D-3 per day.
The safe higher limit of vitamin D-3 changes for younger people. It is 1,000 to 1,500 international units per day for infants and 2,500 to 3,000 international units per day for children, says MedlinePlus. Taking higher doses of vitamin D is not recommended and can lead to vitamin D toxicity, a condition that is characterized by a buildup of calcium in the blood, loss of appetite and nausea, reports Mayo Clinic.