Pharmacokinetics: How the Body Absorbs, Distributes, and Eliminates Drugs

The Body as a Drug Processing Machine

Every drug that enters the human body embarks on a four-stage journey before it reaches its target, exerts its effect, and is finally eliminated. This journey — Absorption, Distribution, Metabolism, and Excretion, universally abbreviated as ADME — defines pharmacokinetics, the mathematical and physiological science of what the body does to a drug. Pharmacokinetics determines not just whether a drug works, but at what dose, how often, by which route, and in which patient populations it remains safe.

Absorption and Bioavailability

Absorption is the process by which a drug moves from its site of administration into the systemic circulation. For intravenous (IV) drugs, this step is bypassed entirely: the drug enters the bloodstream directly, achieving 100% bioavailability by definition. For every other route — oral, intramuscular, subcutaneous, transdermal, inhaled — some fraction of the administered dose is lost before reaching systemic circulation.

Bioavailability (F) is the fraction of an administered dose that reaches the systemic circulation in unchanged, active form. An oral bioavailability of 40% means that if 100 mg is swallowed, only 40 mg reaches the bloodstream in pharmacologically active form. The rest is lost to incomplete absorption through the gut wall, chemical degradation in the gut lumen, or first-pass metabolism in the liver.

First-Pass Metabolism

When a drug is absorbed from the gastrointestinal tract, it travels via the portal vein directly to the liver before entering systemic circulation. The liver — which contains the highest concentration of drug-metabolizing enzymes in the body — can extract and inactivate a significant fraction of the drug on this first pass. This is called first-pass metabolism or the first-pass effect.

Some drugs have remarkably high hepatic extraction ratios. Nitroglycerin, for example, is nearly completely extracted by the liver on first pass, making oral administration essentially useless. It is administered sublingually or transdermally instead, bypassing the portal system. Morphine has an oral bioavailability of roughly 30% due to extensive first-pass metabolism, which is why oral doses must be three times higher than parenteral doses to achieve equivalent analgesia.

Distribution: Where Does the Drug Go?

Once in systemic circulation, a drug distributes throughout the body's fluid compartments and tissues. The volume of distribution (Vd) is a pharmacokinetic parameter that relates the total amount of drug in the body to the plasma concentration at any given time:

Vd = Amount of Drug in Body / Plasma Drug Concentration

A Vd close to plasma volume (~3 L) suggests the drug stays mostly in the bloodstream. A very large Vd — hundreds of liters — indicates extensive tissue sequestration. Chloroquine, an antimalarial, has a Vd of approximately 200–800 L/kg, meaning it accumulates massively in tissues such as the liver, spleen, and kidneys. This is why chloroquine poisoning is so difficult to treat: the drug cannot be simply dialyzed out.

Protein Binding and the Free Drug Fraction

Most drugs circulate in plasma partly bound to proteins, chiefly albumin (for acidic drugs) and alpha-1 acid glycoprotein (for basic drugs). Only the unbound, free fraction of the drug crosses membranes, reaches receptors, and exerts pharmacological effects. Highly protein-bound drugs (e.g., warfarin at ~99% bound) have a very small free fraction. Displacing even 1% of bound drug doubles the free concentration — a clinically meaningful change for narrow-therapeutic-index drugs.

Metabolism: Biotransformation and Clearance

Drug metabolism converts lipophilic, membrane-permeable compounds into more hydrophilic metabolites that can be renally or biliarily excreted. Most metabolism occurs in the liver, primarily through two phases. Phase I reactions (oxidation, reduction, hydrolysis) introduce a reactive functional group, usually via CYP450 enzymes. Phase II reactions (glucuronidation, sulfation, acetylation) conjugate that group to a large polar molecule, producing an easily excreted product.

Hepatic clearance depends on both enzyme capacity and hepatic blood flow. For high-extraction drugs (lidocaine, propranolol), hepatic clearance is primarily flow-limited: anything that reduces liver blood flow — heart failure, cirrhosis, co-administered vasoconstrictors — dramatically reduces clearance. For low-extraction drugs (warfarin, diazepam), clearance depends on intrinsic enzyme activity and is minimally affected by blood flow changes.

Michaelis-Menten vs. First-Order Kinetics

Most drugs at therapeutic concentrations follow first-order kinetics: a constant fraction of the drug is eliminated per unit time, and the rate of elimination is proportional to drug concentration. This produces predictable plasma concentration decay curves. Drug half-life is constant regardless of dose.

A minority of drugs saturate their metabolic enzymes at therapeutic concentrations, following zero-order (Michaelis-Menten) kinetics: a constant amount, not a constant fraction, is eliminated per unit time. Phenytoin and alcohol are classic examples. In zero-order kinetics, small dose increases can cause disproportionate plasma concentration rises, because the elimination machinery is already saturated. This creates a nonlinear dose-response relationship and a steep toxicity cliff.

Half-Life and Steady State

Half-life (t½) is the time required for plasma drug concentration to decrease by 50%. For first-order drugs, half-life is constant and related to volume of distribution and clearance: t½ = 0.693 × Vd / CL

Steady state — the point at which drug input equals drug elimination — is reached after approximately 4–5 half-lives of a drug, regardless of the dosing regimen. A drug with a 12-hour half-life reaches steady state in roughly 2–2.5 days. This has immediate clinical relevance: loading doses are used when steady state must be achieved faster than 4–5 half-lives would allow (e.g., digoxin, amiodarone).

Excretion: Renal Clearance and GFR-Based Dosing

The kidney is the primary excretory organ for most drugs and their metabolites. Renal drug clearance depends on three processes: glomerular filtration, active tubular secretion, and passive tubular reabsorption. Glomerular filtration rate (GFR) directly controls the filtration of unbound drug. In patients with chronic kidney disease, GFR falls and drug clearance decreases, raising plasma concentrations and extending half-lives.

Renal dosing adjustments are calculated using estimated GFR (eGFR), typically derived from creatinine using the CKD-EPI equation. For renally cleared drugs with narrow therapeutic indices — digoxin, aminoglycoside antibiotics, lithium — failing to adjust doses in renal impairment is a leading cause of preventable drug toxicity. GFR matters more than age, weight, or diagnosis for these drugs.

The Therapeutic Index

The therapeutic index (TI) describes the ratio between the toxic dose and the therapeutic dose of a drug. A wide TI means there is a large margin between effective and toxic concentrations. A narrow TI means small changes in plasma concentration can cross from therapeutic to toxic. Narrow-TI drugs demand careful dosing, frequent monitoring, and heightened attention to interactions.

Phenytoin exemplifies the danger of narrow TI combined with Michaelis-Menten kinetics: a dose increase from 300 mg to 400 mg daily can push plasma levels from therapeutic to severely toxic, with no proportional warning. The stakes are high.

Applying Pharmacokinetics Across Patient Populations

Pharmacokinetic parameters are not fixed biological constants — they vary substantially across patient populations. Neonates have immature hepatic enzyme systems and lower renal function, requiring lower weight-adjusted doses of many drugs. Pregnant women have expanded plasma volume, altered protein binding, and accelerated renal clearance — altering the pharmacokinetics of every drug class. Obese patients require dose adjustments based on whether a drug distributes into adipose tissue or lean body mass.

Elderly patients experience parallel declines in hepatic blood flow (25–40% reduction by age 65), renal clearance (GFR declines ~1 mL/min/year after age 40), and plasma albumin — all of which reduce drug clearance and raise effective plasma concentrations. The same dose that is therapeutic in a 40-year-old can be toxic in an 80-year-old. Pharmacokinetics is not one-size-fits-all.

This article is for educational purposes only and does not constitute medical advice. Drug dosing decisions should always be individualized based on clinical assessment by a qualified healthcare provider.