mg is the written short form of milligram or milligrams — a very small unit of weight. One milligram is one thousandth of a gram, so you see it on medicine packets and food labels for tiny amounts: the dose in a pill, a little salt, a vitamin. You write it with small letters, and you do not add an s for plural: 5 mg, not 5 mgs. Out loud you usually say “milligram(s),” not “em-gee,” unless you are spelling the letters. Though actually, students often mix mg with ml (millilitre, a drop of liquid) and with g (gram). 500 mg is a small pill; 500 g is about a loaf of bread.