Hydroxylation

Hydroxylation is a chemical reaction in which a hydroxyl group (−OH) is added to a drug molecule, typically in the liver. It is one of the most common Phase I metabolic reactions — the first stage of biotransformation, in which the body chemically modifies a substance before it can be excreted.

The immediate effect of adding −OH is to make a lipid-soluble molecule more water-soluble, so the kidneys or gut can clear it. Hydroxylated metabolites are often then processed further in a second metabolic stage, where larger molecular groups are attached to carry them out of the body entirely.

How it works · its role

The reaction is carried out mainly by cytochrome P450 (CYP) enzymes — a large family of proteins embedded primarily in liver cells, though also present in the intestinal wall, lungs, and other tissues. CYP enzymes use molecular oxygen and a cofactor (NADPH) to insert one oxygen atom into the target molecule, forming the hydroxyl group.

Hydroxylation can occur on an aromatic ring or on a carbon chain, and different CYP subtypes — CYP3A4, CYP2D6, CYP2C9, and others — specialise in different molecular targets. Individuals vary considerably in how active their CYP enzymes are, due to genetics, age, liver health, and competing substances in the body. The same dose can therefore produce very different metabolite profiles in different people.

The resulting metabolite is usually pharmacologically less active than the parent compound. Occasionally the hydroxylated form is equally or more potent; in rarer cases, hydroxylation generates a reactive intermediate that is itself chemically harmful.

Relevance to substances & effects

Hydroxylation shapes the potency, duration, and risk profile of many substances discussed across these pages.

Cannabis provides a well-known example. When THC is consumed orally, the liver converts it to 11-hydroxy-THC (11-OH-THC) via hydroxylation. This metabolite crosses the blood-brain barrier more readily and is thought to be more potent than THC itself — which is a significant reason edibles tend to produce a stronger and longer-lasting effect than inhaled cannabis.

Opioids, benzodiazepines, and many stimulants all undergo CYP-mediated hydroxylation to varying degrees, often producing active or partially active metabolites that extend or alter the drug's effect profile. Where someone falls on the spectrum of CYP activity can meaningfully change both duration and intensity at the same nominal dose.

Many drug interactions originate here. Substances that inhibit or induce CYP enzymes alter the rate of hydroxylation, raising or lowering circulating drug levels in ways that cannot be read off the label. These pharmacokinetic interactions are catalogued in the interaction layer throughout the encyclopedia.