Liver enzymes

Liver enzymes are proteins in the liver that catalyze the chemical transformations making up drug metabolism. The largest and most pharmacologically significant family is the cytochrome P450 enzymes (abbreviated CYP) — a set of roughly 60 related proteins, of which a handful do the bulk of the body's drug processing. Key members include CYP3A4, which is involved in metabolising roughly half of all pharmaceutical drugs, alongside CYP2D6, CYP2C9, CYP2C19, and CYP1A2.

Beyond the CYPs, enzymes such as alcohol dehydrogenase, monoamine oxidase (MAO), and glucuronyl transferases process specific substance classes. Together these proteins determine not just how quickly a substance is cleared from the body, but often what form it takes while it acts — and what form it takes when it leaves.

How it works · its role

Drug metabolism in the liver generally proceeds in two phases. Phase I enzymes — primarily the CYPs — chemically modify a substance through oxidation, reduction, or hydrolysis, often inserting or unmasking a reactive chemical site. Phase II enzymes then attach a polar molecule to that site, producing a water-soluble conjugate the kidneys can excrete.

Enzyme activity is not fixed. Some substances induce certain enzymes — upregulating production so that other drugs sharing that pathway are processed faster, shortening their duration and reducing their effect. Others inhibit enzymes, slowing metabolism and allowing substances to accumulate to higher levels than a dose alone would suggest.

Genetics adds a further layer. Inherited variants in CYP2D6, for instance, partition people into poor metabolisers, normal metabolisers, and ultra-rapid metabolisers. For substances that depend on this enzyme, the same dose can produce starkly different effects depending on which variant a person carries.

Relevance to substances & effects

Liver enzyme interactions underlie many of the pharmacokinetic drug–drug interactions flagged across this encyclopedia. A substance that strongly inhibits CYP3A4 can cause other substances cleared by that same enzyme to reach unexpectedly high plasma concentrations, intensifying or prolonging their effects beyond what the dose alone would predict.

Several psychoactive substances are prodrugs: pharmacologically inert until liver enzymes convert them into an active compound. Codeine, for example, requires CYP2D6 to produce morphine; a poor metaboliser experiences little opioid effect, while an ultra-rapid metaboliser may be unexpectedly sensitive to a standard dose.

Many combinations that appear in harm reduction contexts — opioids with benzodiazepines, certain antidepressants alongside stimulants, or substances co-ingested with strong CYP inhibitors — carry elevated risk partly because of how they compete for or alter enzymatic capacity. The specific enzyme a substance uses, and whether it inhibits or induces it, determines the direction and size of any interaction. Those severities are sourced and attributed in the interaction layer rather than here, where the concept itself is defined.