In the metric system, a microgram or microgramme is a unit of mass equal to one millionth (1×10−6) of a gram. The unit symbol is μg according to the International System of Units; the recommended symbol in the United States when communicating medical information is mcg. In μg the prefix symbol for micro- is the Greek letter μ (Mu). In the metric system, a microgram or microgramme is a unit of mass equal to one millionth (1×10−6) of a gram. The unit symbol is μg according to the International System of Units; the recommended symbol in the United States when communicating medical information is mcg. In μg the prefix symbol for micro- is the Greek letter μ (Mu).