In general, the therapeutic dose of vitamin D for babies with vitamin D deficiency is 2,000 to 4,000 units per day, so 800 units per day of vitamin D3 for infants would not be excessive.
The toxic dose of vitamin D varies greatly, and some studies have shown that 4,000 units of vitamin D per day orally for 1 to 3 months can cause toxicity in sensitive children, or 20,000 to 50,000 units of vitamin D per day orally for several weeks can cause toxicity, so for babies who have no vitamin D deficiency, 800 units per day for not too long will not be an overdose in general.
To prevent rickets, it is recommended that newborns be given 400 units of prophylaxis from 2 weeks after birth until 2 years of age. Premature infants, low birth weight infants, and twins are given 800 units, which is changed to a preventive dose after 3 months.
Vitamin D3 supplementation in infants should be done under medical supervision to avoid vitamin D toxicity.