CYP2C9

CYP2C9 (cytochrome P450 2C9) is an enzyme in the liver's cytochrome P450 family — the main set of proteins responsible for breaking down drugs and other foreign compounds in the body. It handles a substantial share of clinically used medications, placing it among the most consequential isoforms for predicting drug interactions.

What makes CYP2C9 practically important is its genetic variability. Two common variants — CYP2C9*2 and CYP2C9*3 — reduce enzyme activity substantially. A meaningful fraction of people, particularly those of European ancestry, carry one or both of these variants and clear CYP2C9 substrates more slowly, meaning the same dose can produce higher and longer-lasting concentrations in the blood.

Where a compound's metabolising isoforms have not been established experimentally, CYP2C9 involvement is sometimes inferred from structurally related compounds — a recognised limitation when predicting individual drug behaviour.

How it works · its role

CYP2C9 is embedded in the endoplasmic reticulum of liver cells, where it uses a haem iron centre and molecular oxygen to oxidise its substrates — typically adding a hydroxyl group to the molecule. This converts lipid-soluble compounds into more water-soluble forms that the kidneys can excrete.

Two classes of co-administered drugs can alter its performance. Inhibitors bind to the enzyme and slow it down, causing substrates to accumulate in the bloodstream. Inducers — certain antibiotics and anticonvulsants among them — upregulate CYP2C9 expression, speeding clearance so that substrates may fall below effective concentrations.

Genetic slow-metaboliser status compounds both effects. Someone who is already a poor metaboliser and then takes a CYP2C9 inhibitor can experience further reduced clearance — sometimes enough to push a narrow-margin drug into toxic territory.

Relevance to substances & effects

Among substances in the harm-reduction context, the most clinically significant CYP2C9 substrates are anticoagulants — particularly warfarin — and common anti-inflammatories. Warfarin's therapeutic window is narrow, so even modest shifts in CYP2C9 activity translate into measurable changes in bleeding or clotting risk.

THC, the primary psychoactive constituent of cannabis, is partly metabolised by CYP2C9 alongside other isoforms. Cannabinoids can also act as CYP2C9 inhibitors, slowing the clearance of co-administered pharmaceuticals. This bidirectional relationship — cannabinoids as both substrates and inhibitors — is one reason cannabis interactions appear on pharmaceutical warnings for anticoagulant users.

Several other psychoactive or recreational substances interact with CYP2C9 indirectly, not as substrates but as inhibitors or inducers that shift the clearance of drugs a person may already be taking. The interaction layer of the encyclopedia records these pairings with their severity classifications and evidence provenance.