How Your Body Processes a Medicine

A swallowed immediate release tablet often produces a level that rises and then falls. That is only one pattern. Injections, infusions, patches, inhalers and modified release products behave differently. Pharmacokinetics explains these differences and why dosing instructions must be medicine specific.

Pharmacokinetics describes exposure over time Pharmacokinetics describes what the body does to a medicine over time. It includes absorption, distribution, metabolism and excretion, often shortened to ADME. Pharmacodynamics describes what the medicine does to the body or to a microorganism. The two are connected, but they answer different questions. Pharmacokinetics helps explain how much active medicine reaches relevant sites, how long exposure lasts and why people may need different doses. There is no single medicine concentration curve A swallowed immediate release tablet often dissolves, is absorbed and produces a blood concentration that rises before it falls. This pattern is useful, but it is not universal. An intravenous dose enters the bloodstream without an absorption step. A continuous infusion may build or maintain exposure while it runs. A patch or modified release product can deliver medicine gradually. Some creams, inhalers and swallowed medicines act mainly near where they are given, although some may still enter the bloodstream. The route and formulation therefore shape the concentration pattern. They also influence how quickly an effect begins and how long it may last. Absorption and bioavailability Absorption is movement from the site of administration into the bloodstream. It applies to routes such as oral, inhaled, subcutaneous and transdermal delivery. Bioavailability describes the proportion of a dose that reaches the systemic circulation. An intravenous dose has complete bioavailability, while an oral dose may be absorbed incompletely or changed before reaching the wider circulation. Food may increase, reduce or delay absorption, depending on the medicine and formulation. Food instructions may also aim to reduce stomach irritation rather than alter absorption. Distribution moves medicine between blood and tissues After entering the circulation, a medicine distributes between blood, body fluids and tissues. The pattern depends on blood flow, tissue binding, protein binding and the medicine's chemical properties. A blood concentration is therefore not the same as the total amount of medicine in the body. Two medicines with similar blood levels can distribute very differently. Distribution can also delay the final fall in blood concentration because medicine may move back from tissues into the blood. Metabolism changes the medicine chemically Metabolism changes a medicine's chemical structure. The liver performs much of this work, but metabolism can also occur in the gut, kidneys, lungs, blood and other tissues. Metabolism often makes a medicine easier to remove. It may reduce activity, but it can also create an active or harmful metabolite. A prodrug is given in a form that needs metabolism to produce much of its intended activity. Reduced metabolism can therefore increase one medicine's exposure but reduce another medicine's activation. Excretion, clearance and half life describe different ideas Excretion removes unchanged medicine or metabolites from the body. Urine is an important route, but bile, faeces and exhaled air also contribute for some medicines. Clearance describes the body's overall efficiency in removing a medicine from the circulation. It does not mean that the medicine disappears completely at one moment. Elimination half life is the time taken for the measured concentration to fall by half during the elimination phase. Half life depends on both clearance and distribution. One half life does not remove the entire medicine. Repeated halving means that smaller amounts can remain for several half lives. Repeated doses can accumulate towards steady state When regular doses are given, some medicine may remain when the next dose arrives. The amount can accumulate until average input and average removal become balanced. This is called steady state. Concentrations can still rise and fall between doses, so steady state does not mean a perfectly constant level. For medicines with stable, predictable kinetics, steady state often takes several half lives. Non linear handling, active metabolites, changing organ function or a modified release formulation can make the pattern more complicated. A clinician may sometimes use a loading dose to reach useful exposure sooner. Loading doses are medicine specific and should not be improvised. Concentration and clinical effect are related but not identical A medicine's effect may begin after its blood concentration rises, or continue after that concentration falls. The delay can reflect distribution to the target, active metabolites or slower changes within cells and tissues. Some medicines produce long lasting effects after the active ingredient has largely left the blood. Others require continuing exposure to maintain their effect. A therapeutic range is a blood concentration range used to guide treatment for selected medicines. It is not a universal range that applies to every medicine. Therapeutic drug monitoring is useful when a measured concentration helps improve safety or effectiveness. Most medicines are monitored through symptoms, clinical measurements or safety tests rather than routine drug levels. Even when a range exists, the result depends on sample timing, the person's condition and other findings. A number alone does not establish benefit or toxicity. Dose frequency depends on more than clearance How often a medicine is given depends on half life, formulation, route, the concentration effect relationship and the consequences of high or low exposure. Practical use and adherence also matter. A fast cleared medicine does not always need frequent dosing if its biological effect lasts longer. A long half life does not automatically make once daily dosing appropriate. Immediate release and modified release versions of the same active ingredient may use different schedules. Crushing or opening a modified release product can alter how quickly the dose becomes available. Individual factors can change exposure Age, pregnancy, body composition, genes, acute illness and interactions can change medicine handling. Kidney or liver impairment matters only when it affects that medicine or an active metabolite. The appropriate response may be a lower dose, a longer interval, closer monitoring, another formulation or no change. Product information and current prescribing guidance determine the safe approach. Rapidly changing kidney or liver function can make estimates less reliable. Recent illness, dehydration and new medicines may therefore prompt reassessment. Missed, delayed and extra doses need medicine specific advice A missed or late dose does not have the same consequence for every medicine. Some medicines are time critical, while others allow a wider interval. Check the patient information leaflet or ask a pharmacist or prescriber. Do not take two doses together unless the medicine specific instructions clearly advise this. A possible overdose or unintended extra dose needs urgent advice even when the person initially feels well. Contact NHS 111 if you are unsure what to do. Call 999 for severe breathing difficulty, a seizure, collapse or unresponsiveness after a medicine. This lesson explains general pharmacokinetics. It cannot determine the correct dose, schedule or monitoring plan for an individual medicine.

Pharmacokinetics describes the time course of medicine exposure, not one universal rise and fall curve. Route, formulation, distribution, metabolism and elimination shape that exposure, while the clinical effect may follow a different time course.