The science of medicinal chemistry involves the design and synthesis of novel drugs based on an understanding of how drugs work in the body at the molecular level. Drugs are chemicals that are normally of low molecular weight and which interact with macromolecular targets to produce a biological response. There are several ways in which drugs can be classified. First, drugs can be classified according to their pharmacological effect – for example, analgesics are drugs which have a pain-killing effect. Second, drugs can be classified depending on whether they act on a particular biochemical process. A third method of classifying drugs is by their chemical structure. Finally, classifying drugs according to their molecular target is the most useful classification as far as the medicinal chemist is concerned, since it allows a rational comparison of the structures involved.