# Pharmacodynamics --- **Pharmacodynamics** is a subset of [[pharmacology]] that takes a look at how the actual drug molecules affect the body. ## Agonists & Antagonists A lot of drugs work work by binding to particular [[receptor proteins|receptor molecules]] at various sites of the body. These can either be **agonists**, which mimic the natural ligand and activate the receptor molecule, or **antagonists** which are similar enough to bind to the receptor, but do not cause activation. >[!info] Double Meaning: Agonist >The term **agonist** can also be used to describe motions for [[musculoskeletal system|skeletal muscle]]. - **Agonists** bind to the receptor molecule and ***activate*** it. - **Antagonists** bind to the receptor molecule and *block* it, preventing any other ligand from binding with it. - **Competitive antagonists** bind so closely with the receptor molecule that it will kick off any ligands that are already bound to it. [[opioid antagonists|Narcan]] is an example of a competitive antagonist. - **Noncompetitive antagonists** block the receptor but anything already bound to it remains. The **affinity** of the molecule makes them attach to the receptor in the first place. A molecule with a strong affinity to the receptor matches with it *really* well and wants to bind to it *really* bad. A molecule with a weak affinity to the receptor doesn't want to bind *that* badly, but it will. As a general rule, *agonists have a higher affinity to the receptor than antagonists* (but of course, not always). With that in mind, you have to be careful on what type of drug you chose, because an agonist will trump an antagonist. ![[agonist and antagonist.png]] ___