Whereas pharmacokinetics deals with the body's absorbtion, distribution and excretion of a drug, pharmacodynamics deals with the interactions and effects that a drug has on the body. The study and understanding of a drug's interactions with the body is a critical component in the development of new drugs. The licencing bodies (Food and Drug Administration(FDA) in the US and Medicines Control Agency (MCA) in the UK) require thorough documentation of a new drug's mechanism of action and interaction before they will allow a drug to be marketed.
Drugs usually produce an effect by binding to or interacting with structurally specific receptor proteins found on the surface of cells. This activates a secondary messenger system within the cell which produces a biochemical or physiological change (e.g. a change in the cytoplasmic calcium ion concentration of muscle cells causes the muscle to relax or contract). The most common receptors are transmembrane receptors linked to guanosine triphosphate binding proteins (G proteins) which activate second messenger systems such as adenylylcyclase (beta adrenoceptors) or the inositoltriphosphate pathway (alpha adrenoceptors).
A drug which binds and causes a complete response is called a full agonist.
A drug which binds and causes less than a complete response is called a partial agonist.
A drug which binds but causes no secondary messenger activation is called an antagonist.
Antagonists work by blocking the binding of the body's own agonists to receptors and thereby preventing them from activating secondary messenger systems.
Pharmacodynamics - the concentration-effect relationship. D.J. Birkett. Aust Prescr 1995;18;102-4
Medicinal Chemistry. Thomas Nogrady. Oxford University Press Inc, USA 1988