Enzyme:CYP3A4

CYP3A4 (cytochrome P450 3A4) is the single most clinically consequential drug-metabolizing enzyme in the human body. It is encoded by the CYP3A4 gene on chromosome 7q22.1 and is expressed at high levels in both the liver and the wall of the small intestine. Approximately half of all medicines now in clinical use are substrates of CYP3A4 to a meaningful degree, a figure unmatched by any other cytochrome P450, and the enzyme's promiscuity, its strong inducibility, and its susceptibility to both reversible and mechanism-based inhibition make it the central player in a disproportionate share of drug-drug interactions.

Tissue distribution

CYP3A4 sits at two places along the path a swallowed medicine takes into the body, and both matter clinically. In the small-intestinal wall, CYP3A4 is by far the dominant cytochrome, accounting for roughly 80% of total intestinal CYP protein, and it metabolizes its substrates during their first pass through the enterocytes before they ever reach the portal circulation. In the liver, CYP3A4 accounts for roughly 30 to 40% of total hepatic CYP protein, again the largest share of any single isoform, and it continues the work of clearance that the gut wall began. For a high-extraction substrate like felodipine or simvastatin, the intestinal contribution dominates first-pass metabolism; for a low-extraction substrate like midazolam given orally, both compartments contribute, while for the same drug given intravenously, only the hepatic compartment is in play. This is why grapefruit juice affects oral midazolam strongly but intravenous midazolam barely at all, a fact that has practical consequences for any clinician trying to anticipate an interaction.

Function and substrate spectrum

CYP3A4 catalyzes oxidation, hydroxylation, N- and O-dealkylation, and a number of less common reactions across an unusually wide chemical-structural space. Its active site is large, flexible, and capable of binding more than one substrate molecule simultaneously, which is the molecular basis for its catholic substrate range. When a medicine's clearance route is not immediately obvious, CYP3A4 is the right first guess.

The table below collects the clinically important CYP3A4 substrates by therapeutic class, with each entry tagged by the contribution CYP3A4 makes to overall clearance: major (CYP3A4 is the predominant route; reversible and mechanism-based interactions are clinically expected), moderate (CYP3A4 contributes meaningfully but other routes carry comparable load), minor (CYP3A4 contributes but other pathways dominate), and partial (one of several substantial routes). The list is curated for clinical relevance and is not exhaustive; see Comprehensive substrate and interaction tables below for the authoritative maintained resources.

near-complete CYP3A4 substrate table (click to expand)

Substrate	Therapeutic class	CYP3A4 contribution	Clinical notes
Alfentanil	Opioid analgesic (anesthesia)	major	A classic CYP3A4 probe substrate; major substrate of CYP3A4 inhibitor interactions in the operating room.
Alprazolam	Benzodiazepine	major	Oversedation risk with CYP3A4 inhibitors; reduced effect with inducers.
Amiodarone	Antiarrhythmic	partial	Mixed CYP3A4 + CYP2C8; amiodarone is itself a moderate CYP3A4 inhibitor (long-tail interaction via desethylamiodarone).
Amlodipine	Calcium channel blocker (dihydropyridine)	major	Hypotension with strong CYP3A4 inhibitors.
Apixaban	Direct oral anticoagulant (Xa)	major	Mixed CYP3A4 + P-gp; FDA labeling cautions against use with strong dual CYP3A4 / P-gp inhibitors or inducers.
Aripiprazole	Atypical antipsychotic	partial	Mixed CYP3A4 + CYP2D6; both routes matter.
Atorvastatin	Statin (HMG-CoA reductase inhibitor)	major	Rhabdomyolysis risk with strong CYP3A4 inhibitors; dose caps apply when co-prescribed with cyclosporine, certain HIV antiretrovirals, etc.
Bortezomib	Proteasome inhibitor (anti-cancer)	moderate	Mixed CYP3A4 + CYP2C19.
Buprenorphine	Opioid partial agonist	major	Increased exposures with CYP3A4 inhibitors.
Buspirone	Anxiolytic (5-HT1A partial agonist)	major	Up to 13-fold AUC increase with strong CYP3A4 inhibitors; one of the largest single-medicine CYP3A4-interaction signals in the literature.
Carbamazepine	Antiepileptic, mood stabilizer	major	Substrate AND strong inducer (auto-induces its own metabolism over the first weeks of therapy).
Cisapride	Prokinetic (withdrawn)	major	Withdrawn from most markets in 2000 because of fatal CYP3A4-inhibitor-driven torsades de pointes (the case-defining example of why CYP3A4 interactions matter).
Clarithromycin	Macrolide antibiotic	major	Substrate AND mechanism-based inhibitor; one of the most clinically dangerous CYP3A4 inhibitors in everyday practice.
Colchicine	Anti-inflammatory (gout, FMF)	major	Mixed CYP3A4 + P-gp; severe and sometimes fatal toxicity with strong CYP3A4 inhibitors, especially clarithromycin.
Cyclophosphamide	Antineoplastic (alkylating)	partial	Mixed CYP3A4 + CYP2B6 (dominant); CYP3A4 contributes to bioactivation.
Cyclosporine	Calcineurin inhibitor (immunosuppression)	major	Narrow therapeutic window. Strong CYP3A4 inhibitor co-prescription routinely produces toxic concentrations; strong induction produces graft loss. Therapeutic drug monitoring is standard.
Darifenacin	Antimuscarinic (overactive bladder)	major	Dose cap with strong CYP3A4 inhibitors.
Diltiazem	Calcium channel blocker (non-dihydropyridine)	major	Substrate AND moderate inhibitor of CYP3A4.
Donepezil	Cholinesterase inhibitor (dementia)	partial	Mixed CYP3A4 + CYP2D6.
Dronedarone	Antiarrhythmic	major	Substrate AND moderate CYP3A4 inhibitor.
Erythromycin	Macrolide antibiotic	major	Substrate AND mechanism-based inhibitor; the original "macrolide interaction" archetype.
Estradiol	Endogenous estrogen / hormone therapy	major	CYP3A4 is one of several oxidative routes; strong inducers reduce oral contraceptive efficacy.
Everolimus	mTOR inhibitor (immunosuppression, oncology)	major	Narrow therapeutic window like cyclosporine; therapeutic drug monitoring standard.
Felodipine	Calcium channel blocker (dihydropyridine)	major	The substrate that revealed the grapefruit-juice interaction (Bailey 1991).
Fentanyl	Opioid analgesic	major	Respiratory-depression risk with strong CYP3A4 inhibitors (clarithromycin, fluconazole, ritonavir).
Finasteride	5-alpha reductase inhibitor (BPH, hair loss)	major	Inducers reduce exposure substantially.
Ibrutinib	BTK inhibitor (oncology)	major	FDA labeling caps dose with strong CYP3A4 inhibitors; avoid with strong inducers.
Imatinib	Tyrosine kinase inhibitor (oncology)	major	Substrate AND moderate CYP3A4 inhibitor; mixed pharmacology.
Indinavir	HIV protease inhibitor	major	Substrate AND moderate inhibitor; nephrolithiasis risk linked to high exposures.
Itraconazole	Triazole antifungal	major	Substrate AND strong CYP3A4 inhibitor; one of the strongest used in clinical interaction studies as a probe inhibitor.
Ivabradine	HCN-channel blocker (heart rate)	major	Avoid with strong CYP3A4 inhibitors.
Ketoconazole	Imidazole antifungal	major	Substrate AND the canonical strong CYP3A4 inhibitor used as the probe in regulatory interaction studies (though clinical use has receded because of hepatotoxicity).
Lopinavir	HIV protease inhibitor	major	Substrate AND inhibitor; co-formulated with ritonavir as a deliberate booster.
Lovastatin	Statin	major	Rhabdomyolysis risk; among the most CYP3A4-dependent statins (along with simvastatin).
Methadone	Opioid (chronic pain, OUD)	major	Mixed CYP3A4 + CYP2B6 + CYP2D6; QT prolongation magnified by elevated exposures.
Midazolam	Benzodiazepine (anesthesia, ICU)	major	Canonical CYP3A4 probe substrate. Used in interaction studies as the standard read-out for in-vivo CYP3A4 activity. Oral midazolam is heavily affected by intestinal CYP3A4 (and grapefruit); IV midazolam is affected only by hepatic CYP3A4.
Nifedipine	Calcium channel blocker (dihydropyridine)	major	Hypotension with strong CYP3A4 inhibitors.
Norethindrone	Progestin (oral contraceptive component)	partial	Strong inducers reduce contraceptive efficacy.
Ondansetron	5-HT3 antagonist (antiemetic)	partial	Mixed CYP3A4 + CYP1A2 + CYP2D6.
Oxycodone	Opioid analgesic	partial	Mixed CYP3A4 (major route to noroxycodone) + CYP2D6 (minor route to oxymorphone).
Paclitaxel	Taxane antineoplastic	major	Mixed CYP3A4 + CYP2C8; both routes matter.
Quetiapine	Atypical antipsychotic	major	Substantial AUC increase with strong CYP3A4 inhibitors; oversedation risk.
Quinidine	Antiarrhythmic / CYP2D6 probe inhibitor	major	Also a strong CYP2D6 inhibitor at therapeutic exposures.
Repaglinide	Meglitinide (oral hypoglycaemic)	partial	Mixed CYP3A4 + CYP2C8.
Rivaroxaban	Direct oral anticoagulant (Xa)	major	Mixed CYP3A4 + P-gp; same caution pattern as apixaban.
Sildenafil	PDE5 inhibitor	major	Strong CYP3A4 inhibitors (eg ketoconazole, ritonavir) raise sildenafil AUC several-fold; dose caps apply.
Simvastatin	Statin	major	Among the most CYP3A4-dependent medicines. Strong inhibitors are functionally contraindicated; rhabdomyolysis risk is the limiting toxicity. The simvastatin-erythromycin and simvastatin-clarithromycin interactions are among the most frequently cited in pharmacy literature.
Sirolimus	mTOR inhibitor (immunosuppression)	major	Narrow therapeutic window; therapeutic drug monitoring standard.
Sufentanil	Opioid analgesic (anesthesia)	major	Same interaction pattern as alfentanil.
Tacrolimus	Calcineurin inhibitor (immunosuppression)	major	Narrow therapeutic window. Heavy CYP3A4 dependence with strong genotype-modulation via CYP3A5 (see Major variants section). Therapeutic drug monitoring standard.
Tadalafil	PDE5 inhibitor	major	Same CYP3A4-inhibitor caution pattern as sildenafil.
Tamsulosin	Alpha-1 blocker (BPH)	major	Hypotension risk with strong CYP3A4 inhibitors.
Testosterone	Endogenous androgen / hormone therapy	major	CYP3A4 is one of several oxidative routes.
Triazolam	Benzodiazepine	major	Oversedation risk with strong inhibitors; effectively contraindicated with ritonavir, clarithromycin, ketoconazole.
Vardenafil	PDE5 inhibitor	major	Same CYP3A4-inhibitor caution pattern as sildenafil and tadalafil.
Venetoclax	BCL-2 inhibitor (oncology)	major	FDA labeling: dose reduction or avoidance with strong CYP3A4 inhibitors during ramp-up because of tumor-lysis-syndrome risk.
Verapamil	Calcium channel blocker (non-dihydropyridine)	major	Substrate AND moderate inhibitor of CYP3A4.
Vincristine	Vinca alkaloid (oncology)	major	Severe neurotoxicity with strong CYP3A4 inhibitors; particularly dangerous in pediatric ALL regimens with azole antifungal co-prescription.

Phenotype categories

CYP3A4 phenotyping is a more contested matter than the phenotyping of CYP2D6 or the CYP2C enzymes, and the two major pharmacogenomics consortia do not agree on it. The Clinical Pharmacogenetics Implementation Consortium (CPIC) does not assign CYP3A4 a metabolizer-phenotype classification; in CPIC's framing the population variability in CYP3A4 activity, though wide (roughly 10- to 30-fold between individuals), is dominated by environmental and physiological factors rather than by frankly inactivating star-allele genetics. The Dutch Pharmacogenetics Working Group (DPWG) takes the other position and does define a CYP3A4 phenotype, built around the decreased-function CYP3A4\*22 allele: a poor metabolizer carries two \*22 alleles, an intermediate metabolizer one, and a normal metabolizer none. The DPWG framework underlies that group's guidance on, for example, quetiapine dosing. The wiki indexes the DPWG phenotype categories at Phenotype:CYP3A4 poor metabolizer, Phenotype:CYP3A4 intermediate metabolizer, and Phenotype:CYP3A4 normal metabolizer. Even under the DPWG framework, the CYP3A4 phenotype is a less settled construct than the CYP2D6 or CYP2C19 phenotypes, and environmental induction and inhibition remain the dominant sources of CYP3A4 variability whatever a patient's \*22 genotype.

Major variants

• *1, reference, fully functional
• *22 (rs35599367), a decreased-function allele associated with reduced CYP3A4 expression. Carriers tend to need lower doses of CYP3A4 substrates such as tacrolimus, some statins, and quetiapine. Clinically relevant but not yet a routine pre-prescription test in most settings.
• Many other rare variants (*2 through more than *30) have been catalogued at PharmVar but most have not been linked to robust clinical effects.

The closely related gene CYP3A5 sits next to CYP3A4 on the same chromosomal locus and produces an enzyme with overlapping but not identical substrate preferences. The CYP3A5\*3 variant is much more clinically consequential than any CYP3A4 polymorphism: most people of European ancestry carry two copies and produce essentially no functional CYP3A5, while many people of African ancestry retain a functional copy. CYP3A5 expressers clear tacrolimus markedly faster than non-expressers, which has become a routine pre-transplant pharmacogenomic consideration. See Enzyme:CYP3A5 for the CYP3A5 story in full; this page is CYP3A4.

Inhibitors

CYP3A4 inhibition produces some of the most clinically dangerous drug-drug interactions in medicine because of the breadth of the substrate list. Inhibitors are classified by both strength and kinetic class.

Strong reversible-competitive inhibitors:

• Ketoconazole (the experimental probe; rarely co-prescribed clinically because of its own hepatotoxicity)
• Itraconazole
• Voriconazole, posaconazole, isavuconazole
• Cobicistat (used deliberately as a pharmacokinetic "booster" of HIV antivirals)
• HIV protease inhibitors as a class

Strong mechanism-based inhibitors:

• Furanocoumarins in grapefruit juice (chiefly bergamottin and 6',7'-dihydroxybergamottin), which covalently inactivate intestinal CYP3A4. The interaction was identified serendipitously by Bailey and colleagues in 1991, who noticed that grapefruit juice given to mask the taste of ethanol in a felodipine pharmacokinetic study produced an unexpectedly large rise in plasma felodipine concentrations. Because the inhibition is covalent, the affected enzyme does not recover on the timescale of grapefruit-juice elimination; activity returns over the de-novo synthesis half-life of intestinal CYP3A4, roughly 24 to 72 hours. The clinical consequence is that timing the dose to avoid the juice (for example, taking a statin at night and drinking the juice in the morning) does not abolish the interaction.
• Clarithromycin and erythromycin (mechanism-based component; azithromycin does NOT cause this interaction)
• Ritonavir at therapeutic exposure (mechanism-based component is part of why it boosts so effectively)

Moderate inhibitors:

• Diltiazem, verapamil (calcium channel blockers that are simultaneously substrates of their target enzyme)
• Fluconazole
• Aprepitant
• Cimetidine

Inducers

CYP3A4 is among the most strongly inducible enzymes in human pharmacology. Induction is mediated principally by the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), which up-regulate transcription of the CYP3A4 gene in response to a wide range of xenobiotic signals.

The clinical archetype is rifampin, which over a roughly two-week induction window can increase CYP3A4 activity to two or three times baseline, with corresponding reductions of 50 to 90% in plasma exposures of CYP3A4 substrates. Other meaningful inducers include carbamazepine, phenytoin, phenobarbital, the antiretroviral efavirenz, and the herbal product St John's Wort. Induction is slow on the way in (days to weeks to plateau) and slow on the way out (days to weeks to decay after the inducer is stopped), and so the period after discontinuing an inducer is a particularly hazardous window in which substrate concentrations can rise rapidly toward toxicity.

Clinical implications, summary

For any medicine that depends materially on CYP3A4 for clearance:

• Strong CYP3A4 inhibitor co-prescribed: anticipate substantial, sometimes dramatic, rises in substrate plasma concentrations. For high-extraction substrates given orally, the rise can be five-fold or more. Either avoid the combination or reduce the substrate dose with monitoring. Note the kinetic-class distinction: reversible inhibition resolves with the inhibitor's elimination, but mechanism-based inhibition (grapefruit, clarithromycin, ritonavir) persists for days after the inhibitor is stopped.
• Strong CYP3A4 inducer co-prescribed: anticipate substantial loss of substrate effect. For medicines with a narrow therapeutic window or essential clinical role (oral contraceptives, transplant immunosuppressants, antiretrovirals), induction often forces a regimen change rather than a dose adjustment.
• Inducer recently stopped: the most dangerous window. Reintroduce substrates only with monitoring; expect concentrations to rise over the next 1 to 3 weeks as CYP3A4 expression returns to baseline.
• Genotype-based pre-prescription dosing is not a routine practice for CYP3A4 itself, in contrast to CYP2D6 (for codeine and tramadol) and CYP2C19 (for clopidogrel). The closest practical exception is CYP3A5 genotyping before tacrolimus dosing in solid-organ transplantation.

Comprehensive substrate and interaction tables

The substrate and interaction tables on this page are curated for clinical relevance, not for completeness. Three authoritative external resources maintain comprehensive lists of CYP3A4 substrates, inhibitors, and inducers, and the wiki recommends them to any reader who needs an exhaustive look-up:

• Flockhart Cytochrome P450 Drug Interaction Table, maintained by the Department of Medicine at Indiana University School of Medicine. The most widely cited clinical-reference cytochrome P450 table; substrate-, inhibitor-, and inducer-tiered, updated regularly. Available at https://drug-interactions.medicine.iu.edu/.
• U.S. Food and Drug Administration Drug Development and Drug Interactions Table, the regulatory-grade list FDA uses for labeling and clinical-trial design decisions. Smaller than Flockhart but every entry is FDA-vetted. Available via the FDA Center for Drug Evaluation and Research clinical drug interaction page.
• PharmGKB, the pharmacogenomics knowledge base hosted at Stanford University; the CYP3A4 gene page indexes substrate-, inhibitor-, and inducer-relationships with their underlying primary literature, and links each gene-drug pair to the CPIC dosing guideline where one exists. Available at https://www.pharmgkb.org/.

For a comprehensive review of CYP3A4 (and the rest of the human cytochrome P450 family) covering regulation, polymorphism, and substrate spectrum in detail, the Zanger and Schwab 2013 review in Pharmacology and Therapeutics remains the standard reference.

See also

• Enzyme:CYP3A5, the related isoform with the more clinically actionable polymorphism
• Enzyme:CYP2D6, Enzyme:CYP2C19, Enzyme:CYP2C9, Enzyme:CYP1A2, Enzyme:CYP2B6
• Phenotype:CYP3A4 poor metabolizer, Phenotype:CYP3A4 intermediate metabolizer, Phenotype:CYP3A4 normal metabolizer (the DPWG CYP3A4 metabolizer phenotypes)
• Tacrolimus, Simvastatin, Midazolam, Cyclosporine (canonical CYP3A4 substrate examples)
• Rifampin, Clarithromycin, grapefruit juice (canonical inducer and inhibitor examples)
• Category:Drug-metabolizing enzymes (when this category is built out)