Pharmacology can be defined as the study of the effect of drugs on the functioning of living systems. Drugs can be synthetic chemical compounds, or derived from plant or animal material, or the product of genetic engineering. Compounds like insulin or thyroxine are endogenous hormones, but can also act as drugs if they are administered as such.

The purpose of Pharmacology is to understand what drugs do to living organisms, and then in particular: how these effects can be put to use therapeutically.