A newborn baby who is breastfed should start taking Vitamin D supplements at approximately two weeks of age. We like to make sure that the latch is established and the baby is breastfeeding well, but after that we do like to introduce Vitamin D, because the Vitamin D in breast milk is not transferred well and absorbed well by the baby, so those babies do need extra Vitamin D.
It's sold over the counter. It's one dropper full, which is one mL, 400 units per day. Babies who are bottle-fed standard cow's milk or soy milk formula or any of the standardized formulas, actually, they are all supplemented with vitamins, so you do not need to give a baby who is on formula vitamins.
That being said, some parents do believe in vitamins, and they like to give supplements. Vitamins for children do not cause harm if used in a small dosage, like the recommended child dosage that's over the counter.
After age one, the baby can take the liquid drops. There are certain name brands, but it's a children multi-vitamin that's over the counter. If your doctor recommends that your baby needs iron, you might give a vitamin supplemented with iron as well.
At four months, sometimes babies need more iron, especially babies who are not taking solids yet. So at four months, some doctors recommend Vitamin D drops with iron or a multi-vitamin with iron for your baby. Those are not written with a prescription, but you should discuss with your doctor what vitamins your baby needs or does not need.
In addition, in different states and different localities, some of the water is fluorinated and some is not, so you need to find out if the water in your child's neighborhood is fluorinated or if you need to give your child a fluoride supplement.
In New York City, the water has fluoride, so we do not give our children fluoride in the drops because that would be excess fluoride, and it would cause staining of the teeth. As a child grows, if your child eats a well-balanced diet, they do not need to take a vitamin. I often say that the more expensive the vitamin, the more expensive the urine because they usually just excrete most of it out.