Pharmacodynamics: How Drugs Act

Key points

• • Pharmacodynamics is the study of what a drug does to the body, most often by binding to a target protein such as a receptor, enzyme or ion channel.
• • An agonist binds a receptor and activates it to produce an effect, while an antagonist binds but does not activate, blocking the natural messenger or an agonist.
• • Potency is how much drug is needed to produce an effect (a more potent drug needs less), while efficacy is the biggest effect a drug can produce.
• • The relationship between the amount of drug and its effect is drawn as a dose-response curve, which is typically S-shaped.
• • A competitive antagonist shifts the dose-response curve to the right and can be overcome by adding more agonist, whereas a non-competitive or irreversible one cannot.
• • The therapeutic index compares the amount that causes harm with the amount that helps, so a small index means the drug is risky and needs careful monitoring.