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Pharmacokinetics
Absorption, Distribution, Metabolism, Excretion
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Pharmacokinetics describes what the body does to a drug after it is given. The four main stages are absorption, distribution, metabolism, and excretion, often shortened to ADME. These processes determine how quickly a drug starts working, how strong its effects are, and how long it stays in the body. Understanding ADME helps students connect drug dosing to real clinical outcomes such as efficacy, toxicity, and drug interactions.
After administration, a drug must enter the bloodstream, move into tissues, be chemically changed, and then leave the body. Absorption depends on route, membrane transport, and blood flow, while distribution depends on perfusion, protein binding, and tissue barriers. Metabolism occurs mainly in the liver and often makes drugs easier to eliminate, though some drugs are activated there. Excretion occurs mainly through the kidneys, and changes in liver or kidney function can strongly alter drug levels.
Key Facts
- Bioavailability: F = amount reaching systemic circulation / administered dose
- Volume of distribution: Vd = amount of drug in body / plasma drug concentration
- Clearance: CL = rate of elimination / plasma drug concentration
- Half-life for first-order elimination: t1/2 = 0.693 x Vd / CL
- Loading dose: Loading dose = target concentration x Vd / F
- Maintenance dose rate: Maintenance dose rate = CL x target concentration / F
Vocabulary
- Bioavailability
- Bioavailability is the fraction of an administered dose that reaches the systemic circulation unchanged.
- First-pass metabolism
- First-pass metabolism is drug metabolism in the gut wall or liver before the drug reaches the systemic circulation.
- Volume of distribution
- Volume of distribution is an apparent volume that relates the amount of drug in the body to its plasma concentration.
- Clearance
- Clearance is the volume of plasma from which a drug is completely removed per unit time.
- Half-life
- Half-life is the time required for the plasma concentration of a drug to fall to half of its current value.
Common Mistakes to Avoid
- Assuming oral dose equals systemic dose, which is wrong because incomplete absorption and first-pass metabolism can reduce the amount reaching circulation.
- Confusing volume of distribution with a real anatomical volume, which is wrong because Vd is a calculated value that reflects how widely a drug leaves the plasma.
- Thinking metabolism always inactivates drugs, which is wrong because some drugs are converted into active metabolites or activated from prodrugs.
- Assuming a long half-life always means poor kidney excretion, which is wrong because half-life depends on both clearance and volume of distribution.
Practice Questions
- 1 A drug is given orally at 200 mg, and 120 mg reaches the systemic circulation unchanged. What is the bioavailability F?
- 2 A drug has Vd = 40 L and CL = 5 L/hour. Calculate its half-life using t1/2 = 0.693 x Vd / CL.
- 3 A patient with severe liver disease is given a drug that normally undergoes extensive first-pass metabolism. Explain how this could change oral bioavailability and why the dose might need adjustment.