The relationship between a drug (ligand) and its receptor is often described using a "lock and key" model.
There are four major types of receptors that facilitate these signals: pharmacodynamic
: Trigger intracellular signal cascades. The relationship between a drug (ligand) and its
: The ratio between the dose that causes toxicity and the dose that produces a therapeutic effect. A "narrow therapeutic index" means there is a slim margin of safety. Factors Influencing Response or ion channels directly. Agonists
Beyond receptors, drugs may also target enzymes (such as NSAIDs inhibiting cyclooxygenase), transporters, or ion channels directly. Agonists, Antagonists, and Affinity
: Drugs that bind to a receptor but do not activate it, effectively blocking other molecules from binding.