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Cytochrome P450 & Drug Interactions

Name: Cytochrome P450 & Drug Interactions
Creator: Veli Bakalov, MD

Inducers, inhibitors, and why the pill list can be lethal

Pharmacology Principles·Drug Metabolism & Interactions·4 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Phase I (CYP450 oxidation, reduction, hydrolysis in smooth ER) yields slightly polar, often active or toxic metabolites and is lost first with aging; phase II conjugation yields very polar, inactive metabolites.
LOT benzodiazepines (lorazepam, oxazepam, temazepam) use only phase II and are preferred in liver disease and the elderly; slow acetylators on isoniazid, hydralazine, or procainamide risk drug-induced lupus.
CYP inducers (chronic alcohol, St. John's wort, phenytoin, phenobarbital, nevirapine, rifampin, griseofulvin, carbamazepine, modafinil) act over days to weeks and LOWER levels of other drugs.
CYP inhibitors (valproate, isoniazid, cimetidine, azoles, acute alcohol, chloramphenicol, macrolides, sulfonamides, ciprofloxacin, omeprazole, metronidazole, amiodarone, ritonavir, grapefruit) act within hours and RAISE levels of other drugs.
Warfarin (CYP2C9, narrow TI): inhibitors such as amiodarone, metronidazole, fluconazole, and TMP-SMX raise INR and cause bleeding; inducers such as rifampin and carbamazepine lower INR.
Grapefruit juice inhibits intestinal CYP3A4, raising simvastatin/atorvastatin levels toward myopathy and rhabdomyolysis; macrolides and azoles do the same.
Inducers (rifampin, phenytoin, carbamazepine, phenobarbital, St. John's wort) cause oral contraceptive failure; counsel backup contraception.
Acute alcohol inhibits CYP; chronic alcohol induces CYP2E1 and depletes glutathione, increasing NAPQI and acetaminophen hepatotoxicity.
Prodrugs need CYP activation: clopidogrel via CYP2C19 (blunted by omeprazole) and codeine to morphine via polymorphic CYP2D6; CYP3A4 metabolizes the most drugs.

Phase I and phase II metabolism

Cytochrome P450. A family of heme-containing oxidase enzymes in the smooth endoplasmic reticulum of hepatocytes (and intestinal epithelium). They make lipophilic drugs more polar so they can be excreted, and they are the main site of drug-drug interactions.

Phase I. Oxidation, reduction, and hydrolysis, run mainly by CYP450. Products are only slightly polar and often still active or even toxic. Phase I capacity declines with age, so the elderly lose it first.

Phase II. Conjugation (glucuronidation, acetylation, sulfation, glutathione) produces very polar, usually inactive, water-soluble metabolites for renal excretion. Acetylation speed is genetic: slow acetylators taking isoniazid, hydralazine, or procainamide are at risk of drug-induced lupus.

Phase I vs phase II

Feature	Phase I	Phase II
Reactions	Oxidation, reduction, hydrolysis (CYP450)	Conjugation: glucuronide, acetyl, sulfate, glutathione
Products	Slightly polar; often active or toxic	Very polar; usually inactive, renally excreted
With aging	Lost first	Relatively preserved
Clinical hook	Site of most interactions	Slow acetylators → drug-induced lupus

LOT benzodiazepines (Lorazepam, Oxazepam, Temazepam) undergo only phase II conjugation, so they are the safer choices in liver disease and in the elderly.

Inducers and inhibitors

Enzyme inducers. Increase synthesis of CYP450 enzymes (by activating nuclear receptors that raise gene transcription), so the effect builds over days to weeks. Metabolism of co-administered drugs speeds up and their levels FALL, making them subtherapeutic: oral contraceptive failure and warfarin resistance are the classic results. Stopping an inducer does the reverse, and levels of the other drug rise as induction wears off. Carbamazepine even induces its own metabolism (autoinduction).

Classic inducers. Chronic alcohol, St. John's wort, phenytoin, phenobarbital, nevirapine, rifampin, griseofulvin, carbamazepine (and modafinil). Mnemonic: chronic alcoholics Steal Phen-Phen and Never Refuse Greasy Carbs. Rifampin is the prototypical potent inducer: it "RAMPS up" metabolism.

Enzyme inhibitors. Block CYP450 directly, so the effect appears quickly (often within hours). Metabolism of co-administered drugs slows and their levels RISE toward toxicity: a high INR with bleeding when added to warfarin, or statin myopathy.

Classic inhibitors. Sodium valproate, isoniazid, cimetidine, ketoconazole and other azoles (fluconazole), acute alcohol, chloramphenicol, erythromycin/clarithromycin, sulfonamides, ciprofloxacin, omeprazole, metronidazole (SICKFACES.COM), plus amiodarone, ritonavir, and grapefruit juice.

Inducers vs inhibitors

Feature	Inducers	Inhibitors
Effect on CYP	↑ Enzyme synthesis	↓ Enzyme activity
Onset	Days to weeks	Hours
Levels of other drugs	Fall (subtherapeutic)	Rise (toxicity)
Classic consequence	OCP failure, low INR and clotting	High INR and bleeding, statin rhabdomyolysis
Prodrugs needing CYP	More activation	Less activation, lower efficacy
Examples	Rifampin, carbamazepine, phenytoin, phenobarbital, St. John's wort, griseofulvin, chronic alcohol	Azoles, macrolides, cimetidine, amiodarone, ritonavir, grapefruit, isoniazid, metronidazole

Classic clinical interactions

Warfarin, the interaction magnet. Metabolized by CYP2C9 and has a narrow therapeutic index. Inhibitors (amiodarone, metronidazole, fluconazole, TMP-SMX) raise the INR and cause bleeding; inducers (rifampin, carbamazepine) lower the INR and risk clotting. Check the INR whenever a medication is started or stopped.

Grapefruit and statins. Grapefruit juice inhibits intestinal CYP3A4, increasing the bioavailability of many drugs. With simvastatin or atorvastatin it raises the risk of myopathy and rhabdomyolysis; it also raises levels of some calcium channel blockers and immunosuppressants. Any statin plus a CYP3A4 inhibitor (grapefruit, azoles, macrolides) is a classic rhabdomyolysis setup.

Oral contraceptive failure. Inducers accelerate estrogen and progestin metabolism, lowering hormone levels enough to allow pregnancy. Culprits: rifampin, phenytoin, carbamazepine, phenobarbital, and St. John's wort; counsel backup contraception.

Alcohol: acute inhibits, chronic induces. Acute intoxication inhibits CYP by competing for the enzymes, while chronic use induces CYP2E1. Acetaminophen is normally cleared mostly by phase II conjugation, with a small fraction oxidized by CYP2E1 to the toxic NAPQI, which glutathione neutralizes. CYP2E1 induction plus glutathione depletion explains acetaminophen hepatotoxicity at lower doses in chronic drinkers.

AcetaminophenMostly glucuronidated and sulfated (phase II)
CYP2E1 fractionOxidized to NAPQI (phase I)
GlutathioneConjugates and detoxifies NAPQI
Chronic alcoholMore CYP2E1 and less glutathione → hepatocyte necrosis

Why induction plus glutathione depletion is dangerous

Prodrugs flip the logic. When CYP activates a drug, inhibitors reduce efficacy. Clopidogrel is activated by CYP2C19, and omeprazole can blunt it. Codeine is converted to morphine by CYP2D6, so CYP2D6 genotype changes both analgesia (poor metabolizers get little relief) and toxicity (ultrarapid metabolizers risk respiratory depression).

Workhorse isoforms

High-yield CYP isoforms

Isoform	Key substrates	Interaction notes
CYP3A4	The most drugs: statins, calcium channel blockers, cyclosporine and tacrolimus	Inhibited by grapefruit, azoles, macrolides, ritonavir; induced by rifampin
CYP2D6	Codeine, many antidepressants and antipsychotics	Highly polymorphic: poor to ultrarapid metabolizers
CYP2C9	Warfarin, phenytoin	Narrow-TI substrates; inhibitors cause toxicity
CYP2C19	Clopidogrel (activation)	Omeprazole or loss-of-function variants blunt effect
CYP2E1	Acetaminophen (to NAPQI), ethanol	Induced by chronic alcohol

Genetic variation in these isoforms changes drug response independent of interactions.

Predict the direction first: inducer → less drug effect (unless a prodrug); inhibitor → more drug effect and toxicity (unless a prodrug). The dangerous substrates are the narrow-therapeutic-index ones: warfarin, phenytoin, and immunosuppressants.