Non-clinical sciences/Pharmacology principles · Drug metabolism

CYP450 inducers

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What it means

Increase CYP enzyme synthesis over days–weeks → faster metabolism → LOWER levels of substrates (warfarin, COC, DOACs, ciclosporin, antiretrovirals). Mnemonic PC BRAS: Phenytoin, Carbamazepine, Barbiturates, Rifampicin, Alcohol (chronic), Sulfonylureas/Smoking, St John's wort.

Worked example

Rifampicin induces CYP → contraceptive-pill failure; advise additional/alternative contraception.

In the exam

Starting rifampicin lowers a woman's ethinylestradiol levels over the following weeks, risking contraceptive failure.

What settles it

Inducers LOWER substrate levels (effect builds over days–weeks via new enzyme); inhibitors RAISE them (within hours).

Classically confused with

CYP450 inhibitors

Source: BNF — Interactions