Back in 2008, the American Academy of Pediatrics began recommending supplementing vitamin D in breast fed infants. The original report is here. For quite some time, the only form of vitamin D supplement was a drop with the vitamin D combined with vitamin A and C (TriViSol or ViDaylin ADC). However, newer forms of drops are available that allow us to just do vitamin D by itself. The recommendation is for 400 IU (international units) each day. Enfamil D-Vi-Sol, with 400 IU per ml, is available and is 1 ml by mouth once a day. The other option is Baby D Drops, which are 400 IU per drop. Therefore the dose for the Baby D Drops is 1 drop by mouth once a day.
If a baby takes formula, it is 16 ounces per day that would supply 400 IU in a day. Therefore, if an infant is breast feeding but supplements with formula, it is 16 ounces a day that are needed to supplement to skip doing the vitamin D drops.
We do recommend supplementing breast fed (or breast fed plus formula supplemented infants who take less than 16 ounces of formula per day) infants with vitamin D.