Pharmacokinetics Quiz

Pharmacokinetics is the study of what the body does with a drug. It covers four core processes summarized by the acronym ADME: absorption, distribution, metabolism, and excretion. These processes together determine how much of a drug reaches its target site and for how long.

Pharmacokinetics describes what the body does with a drug, including how it is absorbed, distributed, metabolized, and excreted. Pharmacodynamics describes what the drug does to the body, such as its mechanism of action and therapeutic effects. The two fields are closely linked because pharmacokinetic processes determine the drug concentrations that drive pharmacodynamic responses.

ADME stands for absorption, distribution, metabolism, and excretion. Absorption covers how a drug enters systemic circulation, including factors like passive diffusion and bioavailability. Distribution refers to how the drug spreads into body fluid compartments, metabolism involves enzymatic biotransformation (often in the liver), and excretion is primarily renal filtration plus secretion minus reabsorption.

Metabolism in pharmacokinetics, also called biotransformation, is the enzymatic chemical alteration of a drug, mainly in the liver. Phase I reactions functionalize the drug to form metabolites, while Phase II reactions conjugate those metabolites for excretion. Importantly, biotransformation does not always inactivate a drug, as some metabolites remain pharmacologically active, and others, like the reactive metabolite of paracetamol, can be toxic.

Bioavailability is the fraction of an administered dose that reaches systemic circulation intact. Oral drugs are particularly affected by the first-pass (hepatic) effect, where the liver metabolizes a drug before it enters general circulation, reducing the amount available to act on target tissues. Intravenous administration bypasses this effect and achieves 100% bioavailability.

Urine pH influences how much of a drug is ionized in the renal tubule, which in turn affects reabsorption. Ionized drug molecules cannot easily cross membranes, so they stay in the tubule and are excreted. For example, acidifying the urine increases the ionized fraction of basic drugs, reduces their reabsorption, and increases their clearance.