Pharmacodynamics explains how drugs affect the body and how the size of a dose relates to the size of a response. This cheat sheet helps students read dose-response graphs, compare drugs, and understand important safety measures. It is useful for connecting biology, chemistry, and medical science concepts in a clear reference format.
The most important ideas are potency, efficacy, EC50, ED50, TD50, and therapeutic index. A graded dose-response curve shows how one patient or tissue responds as dose increases, while a quantal curve shows the percent of a population that reaches a defined effect. Safer drugs usually have a wider therapeutic window and a larger therapeutic index.
Key Facts
- Emax is the maximum effect a drug can produce, no matter how much more dose is given.
- EC50 is the concentration that produces 50% of the drug's maximum effect in a graded dose-response curve.
- ED50 is the dose that produces the desired effect in 50% of a population in a quantal dose-response curve.
- TD50 is the dose that produces a toxic effect in 50% of a population.
- Therapeutic index is calculated as TI = TD50 / ED50, and a larger TI usually means a safer drug.
- Potency compares how much drug is needed for an effect, so a lower EC50 or ED50 means higher potency.
- Efficacy compares the maximum effect of drugs, so the drug with the higher Emax has greater efficacy.
- A competitive antagonist shifts the dose-response curve to the right and increases EC50 without lowering Emax if enough agonist is present.
Vocabulary
- Pharmacodynamics
- The study of what a drug does to the body, including its effects, mechanisms, and dose-response relationship.
- Dose-response curve
- A graph that shows how the response to a drug changes as the dose or concentration increases.
- Potency
- A measure of how much drug is needed to produce a given effect, often compared using EC50 or ED50.
- Efficacy
- The maximum effect a drug can produce, often represented by Emax on a dose-response graph.
- Therapeutic window
- The range of drug doses or concentrations that is likely to be effective without causing unacceptable toxicity.
- Agonist
- A drug or molecule that binds to a receptor and activates it to produce a biological response.
Common Mistakes to Avoid
- Confusing potency with efficacy is wrong because potency describes dose needed, while efficacy describes maximum effect.
- Assuming a higher dose always gives a stronger response is wrong because responses can plateau at Emax when receptors or pathways are saturated.
- Reading EC50 as the dose for 50% of people is wrong because EC50 applies to a graded response, while ED50 applies to a population response.
- Treating a small therapeutic index as safer is wrong because TI = TD50 / ED50, and a small value means toxic and effective doses are close together.
- Ignoring the axis scale on dose-response graphs is wrong because many pharmacology graphs use a logarithmic dose axis, which changes how spacing should be interpreted.
Practice Questions
- 1 Drug A has an EC50 of 2 mg/L and Drug B has an EC50 of 10 mg/L. Which drug is more potent, and why?
- 2 A medication has an ED50 of 25 mg and a TD50 of 200 mg. Calculate the therapeutic index using TI = TD50 / ED50.
- 3 On a graded dose-response curve, Drug X reaches an Emax of 90 units and Drug Y reaches an Emax of 60 units. Which drug has greater efficacy?
- 4 A competitive antagonist shifts an agonist dose-response curve to the right without lowering Emax. Explain what this means for potency and maximum effect.