Pharmacodynamics describes what the drug does to the body. It is a detailed study of how drugs act and tries to answer the question whether a drug provides a meaningful pharmacological action. Note that with drugs we usually mean the active pharmaceutical ingredient (API) of a formulation (that can be any formulation like tablet, spray, cream, injection liquid, et cetera), not the other components (e.g. sweeteners, starch, preservatives, etc).
Quantitative methods and mathematical analyses are often used in pharmacodynamic studies to compare the effects of drugs. The results of such studies are often presented in the form of a graph as a dose-response curve.
The introduction of the concept "receptor" at the end of the nineteenth century was of great importance in pharmacodynamics. Drugs do not have an effect unless they bind to a receptor. However, this is not true for all drug actions. Additionally, binding not always means that an activating response is initiated. Many drugs inhibit or block a response that is already present.
This chapter will first describe the various types of interactions that a drug can have at a receptor. We then continue with describing the various targets that drugs can have effects on. Next, we focus more in depth on the interactions that drugs have at these targets. Finally, we elaborate on signalling systems in humans, and how drugs interfere with these systems by illustrating this with the nervous system and the inflammatory system.