Medical chemistry is a discipline of medicine at the intersection of chemistry, biochemistry, pharmacology, toxicology and various other biological specialties, where it is involved with design, chemical synthesis, biochemical analysis, and development for market of pharmaceutical agents, or bio-active molecules. 
In particular, medical chemistry in its most common practice — focusing on small organic molecules, encompasses synthetic organic chemistry and aspects of natural products and computational chemistry in close combination with chemical biology, enzymology and structural biology, together aiming at the discovery and development of new therapeutic agents. It includes synthetic and computational aspects of the study of existing drugs and agents in development in relation to their bioactivities (biological activities and properties), i.e., understanding their quantitative structure-activity relationships (QSAR).
Recommended reading and video lectures (only for registered users):
Enjoy! 
Dr. Narcyz Knap, MD PhD
Department of Medical Chemistry
Medical University of Gdansk, Poland
