Elimination

Elimination is the final stage of a substance's journey through the body — the period in which its concentration in blood and tissues falls, as the body converts and removes it. It follows absorption and distribution and is almost always discussed in terms of the elimination half-life: the time it takes for the plasma concentration of a compound to drop by half.

The concept covers two distinct but usually simultaneous processes: metabolism (chemical transformation, primarily in the liver) and excretion (physical removal through the kidneys, bile, lungs, or sweat). These are treated together because both reduce circulating concentration, and it is their combined rate that determines how long a substance remains active.

How it works · its role

Most substances are metabolised primarily in the liver, where enzyme families — especially the cytochrome P450 group — chemically modify the compound, often rendering it inactive or water-soluble enough to be filtered by the kidneys. This is called Phase I metabolism. Phase II reactions then attach molecular groups to the compound or its Phase I product, further easing its removal.

For most compounds, elimination follows first-order kinetics: a constant proportion of whatever remains is cleared per unit time, so the declining concentration traces an exponential curve. Ethanol at intoxicating doses is a well-known exception — it follows zero-order kinetics, clearing at a roughly constant amount per hour regardless of how much is present.

Excretion adds further routes. The kidneys filter blood continuously, and compounds water-soluble enough pass into urine. Some substances are excreted in bile and leave via the gut. A smaller fraction exits through the lungs — the basis of breathalyser tests — or, to a minor extent, through sweat and saliva.

Relevance to substances & effects

Half-life is one of the most practically meaningful numbers on a substance page. A compound with a short half-life — a few hours or less — peaks quickly and clears quickly; its effects are intense but brief. A compound with a long half-life lingers for days, producing sustained effects and a higher risk of accumulation with repeated dosing.

Many sedatives and opioids complicate this picture by producing active metabolites — breakdown products that are themselves pharmacologically potent — which can extend effects well beyond the apparent duration of the parent compound. Benzodiazepines vary considerably here: some generate long-lived active metabolites, making their effective half-life much longer than the parent drug's figure alone suggests.

Elimination also sits at the centre of many clinically significant drug interactions. When two substances compete for the same metabolic enzymes, one can slow the clearance of the other, raising its concentration and amplifying or prolonging its effects. This mechanism underlies a number of the dangerous combinations flagged across the encyclopedia, and it is why a substance's metabolic pathway is noted wherever it is known.