Forensic science is the parent subject of many others, including forensic psychology! Forensic medicine only deals with the human body, as Ayesha mentioned, determining the cause of death and similar stuff. In order to practice forensic medicine, you have to be a medical practitioner first.
Forensic science is the use of scientific methods to investigate crimes. While on the other hand forensic medicine is a branch of forensic science. It applies medical knowledge to criminal and civil laws. For instance: determining cause of death, blood work in paternity test and etc.