A medicine’s journey through the body.

Absorption: getting into circulation

After you swallow a tablet, its active ingredient must be released, dissolve, cross the gut wall and reach the bloodstream. Food, stomach emptying, acidity, the formulation, transport proteins and interactions can change how quickly or how much is absorbed.

For a medicine intended to act throughout the body, bioavailability is often described as the fraction of the dose that reaches the circulation unchanged. Regulatory definitions also consider how quickly and how much becomes available at the site of action, including for products intended to act locally. An oral substance may be metabolised in the intestinal wall or liver before reaching wider circulation: the first-pass effect. The route and formulation matter; this page explains concepts rather than how to administer a product.14510

Distribution: reaching different places

Circulating blood carries substances around the body, but tissues do not all receive the same exposure. Molecules may bind to plasma proteins, enter some tissues more readily than others, or encounter barriers such as the blood–brain barrier. Blood concentration is a practical measurement, not a complete map of where a substance is. The apparent volume of distribution is calculated from the amount of drug in the body and its concentration in plasma. It is not necessarily the literal volume of space the drug occupies.167

Metabolism and excretion: changing and leaving

Enzymes in the liver, intestine and other tissues can chemically alter a medicine. Some changes reduce activity. Others produce active metabolites (breakdown products that still have an effect) or activate a prodrug (a medicine that needs conversion to become active). The body removes parent drugs and metabolites through the kidneys, bile, faeces, lungs and other routes. Clearance summarises how the body removes a drug under a specified model. These processes overlap rather than follow four neat sequential stages, and organ function, age, genetics, food and interacting medicines can change them.138

Exposure can change while the dose stays the same

A medicine’s pharmacokinetics are not always fixed. One medicine can inhibit an enzyme or transporter and raise another medicine’s exposure; an inducer can increase metabolic or transport capacity and lower it. The change may be delayed, may differ by organ or pathway, and may persist after the interacting medicine is stopped. Food, inflammation, organ impairment, formulation changes and genetic differences can also alter exposure. Regulatory interaction studies therefore ask both whether a medicine is a victim of another product’s effect and whether it is a perpetrator that changes another product’s pharmacokinetics. An interaction prediction or in-vitro result is not automatically the size of the effect in a patient: clinical studies, modelling and the product label determine how far the evidence can be applied.118

Follow the concentration–time curve

Researchers often plot plasma concentration against time. The curve may rise during absorption, reach a maximum and decline as distribution and elimination proceed. Maximum concentration describes a peak, time to maximum describes when that peak was observed, and area under the curve summarises exposure over a defined interval. They answer different questions and none is automatically a measure of clinical benefit. A peak can matter for tolerability; total exposure can matter for efficacy or toxicity; a trough can matter when concentrations must remain above a threshold. The relevant measure depends on the medicine, target, outcome and study design.

When a drug remains between repeated doses, concentrations can accumulate toward a repeated-dose pattern in which average input and elimination balance. For a stable, linear dosing regimen, effective half-life describes the time course of accumulation toward that pattern. A terminal half-life may describe a late concentration phase and need not represent the accumulation time when a drug has several distribution phases. Delayed absorption, active metabolites or nonlinear kinetics can complicate the simple model; dose interval and formulation also affect peaks and troughs. A blood curve is therefore evidence about exposure, not a direct picture of receptor occupancy or a guarantee of a particular response.97

Parent drug, metabolites and measurement

A concentration result should identify what was measured. A laboratory may measure the parent compound, one or more metabolites, or total and unbound concentrations. A metabolite can be inactive, active, longer-lived or associated with a different tissue distribution. Protein binding can make total concentration and unbound concentration tell different stories. Sampling time also matters: a result taken near a peak cannot be compared casually with one taken during a trough. Pharmacokinetic models help estimate parameters such as clearance and distribution from sparse samples, but estimates depend on the sampling plan, assay, model and assumptions.

An exposure study can answer whether two formulations or routes produced similar concentrations under the tested conditions. It does not by itself prove equivalent clinical outcomes, long-term safety or interchangeability in every population. The reader should keep the formulation, route, dose, timing, analyte and study population attached to every number.379

Why this changes interpretation

An effect in a laboratory dish does not demonstrate that the relevant exposure can be achieved safely in a person. A single dose in healthy volunteers cannot automatically represent repeated dosing in people with a disease. Kidney or liver impairment, age, body composition, pregnancy, genetics, adherence and interactions can change exposure. The journey from an administered product to its biological target is part of what development must investigate.28

Five questions to carry forward

What route, formulation, dose and timing were studied? Was exposure measured as concentration, maximum concentration or area under the curve? Which parent compound and metabolites were measured? What model was used for clearance, distribution and half-life? Which people were studied, and could organ function, interactions or adherence change the result?39