Pharmacology (from Greek lang: φάρμακον, pharmakon, "drug"; and lang: -λογία, -logia) is the study of drug action. More specifically it is the study of the interactions that occur between a living organism and exogenous chemicals that alter normal biochemical function. If substances have medicinal properties, they are considered pharmaceuticals. The field encompasses drug composition and properties, interactions, toxicology, therapy, and medical applications and antipathogenic capabilities. Pharmacology is not synonymous with pharmacy, which is the name used for a profession, though in common usage the two terms are confused at times. Pharmacology deals with how drugs interact within biological systems to affect function. It is the study of drugs, of the body's reaction to drugs, the sources of drugs, their nature, and their properties. In contrast, pharmacy is a medical science concerned with the safe and effective use of medicines.

The origins of clinical pharmacology date back to the Middle Ages in Avicenna's The Canon of Medicine, Peter of Spain's Commentary on Isaac, and John of St Amand's Commentary on the Antedotary of Nicholas. Pharmacology as a scientific discipline did not further advance until the mid-19th century amid the great biomedical resurgence of that period. H P Rang. (2006) The receptor concept: pharmacology's big idea. Br J Pharmacol. 147 Suppl: S9–S16. Before the second half of the nineteenth century, the remarkable potency and specificity of the actions of drugs such as morphine, quinine and digitalis were explained vaguely and with reference to extraordinary chemical powers and affinities to certain organs or tissues. Andreas-Holger M., Cay-Rüdiger P. and R. F. Halliwell (2002), The emergence of the drug receptor theory. Nature Reviews Drug Discovery 1, 637-641 The first pharmacology department was set up by Buchheim in 1847, in recognition of the need to understand how therapeutic drugs and poisons produced their effects.

Early pharmacologists focused on natural substances, mainly plant extracts. Pharmacology developed in the 19th century as a biomedical science that applied the principles of scientific experimentation to therapeutic contexts.

Divisions

Pharmacology as a chemical science is practiced by pharmacologists. Subdisciplines include

• clinical pharmacology - the medical field of medication effects on humans
• neuro- and psychopharmacology (effects of medication on behavior and nervous system functioning),
• pharmacogenetics (clinical testing of genetic variation that gives rise to differing response to drugs)
• pharmacogenomics (application of genomic technologies to new drug discovery and further characterization of older drugs)
• pharmacoepidemiology (study of effects of drugs in large numbers of people)
• toxicology study of harmful effects of drugs
• theoretical pharmacology
• posology - how medicines are dosed
• pharmacognosy - deriving medicines from plants