So, your baby's doctor has prescribed Vitamin D supplements, but is it really necessary and which kind should you get?
Vitamin D supplements traditionally come in a syrup, and one dose is a syringe full of extra things your baby might not need and could actually cause gas and fussiness (like sugar)! They usually also contain Iron which, in supplement form, is hard for babies to digest, feeds the bad gut bacteria, and ends up causing uneccesary fussiness. Breastmilk contains all the iron most healthy babies need until they are 6 months old. (Source: http://kellymom.com/nutrition/vitamins/iron/)
Instead of the large dose, consider finding a Vitamin Supplement that contains a full dose (400IU) in one drop. Examples of these are Carlson's and BabyD. They take the mess and the additives out of the equation, they are easy to use, and the bottle lasts for 3 months to a year depending on the quantity.