Pharmacodynamics describes how a medication or supplement interacts with the body to produce a particular effect, including both therapeutic and adverse effects. Simply put, pharmacodynamics looks at what a medication does to the body, as opposed to pharmacokinetics, which focuses on what the body does to a medication.^[1]

In general, drugs produce an effect by interacting with molecules in the body, either by interacting with a target site in the body (e.g., a receptor, enzyme, ion channel, carrier protein) to enhance, reduce, stabilize, or block its function, or by participating directly in chemical reactions with various biological molecules.^[1] The pharmacodynamics of a particular drug are determined by four things: its unique mechanism of action, the concentration of the drug at its target site (the dose-response relationship), how tightly it binds to its target site, and the characteristics of the person taking the drug (e.g., disease states, genetics, age).^[1]

A pharmacodynamic drug interaction can occur in an additive, synergistic, or antagonistic way. Additive interactions occur when two compounds with a similar effect are combined, leading to an increased total effect that is roughly equal to the sum of the individual effects. A synergistic interaction occurs when the combination of two compounds leads to an increase in effect that exceeds the sum of the individual effects. Lastly, an antagonistic interaction occurs when two compounds with opposing effects are combined, leading to a reduction in the effect of both compounds. ^[2]