The CYP450 Report
Section 1: Introduction -- The Invisible Interaction
A 54-year-old woman in Kingston has atrial fibrillation managed with warfarin 5 mg daily. Her INR has been stable at 2.4 for eight months. She develops a respiratory tract infection and is prescribed clarithromycin 500 mg twice daily for seven days.
On day five, she presents to the emergency department with haematuria and a nosebleed. Her INR is 6.8.
Nothing in her warfarin dose has changed. Her diet has not changed. She has not started any other prescription medication. The only new drug is the clarithromycin -- and clarithromycin is a potent inhibitor of CYP3A4, one of the enzymes responsible for warfarin metabolism. With the enzyme blocked, warfarin accumulates. The INR rises. The patient bleeds.
This interaction is predictable. It is documented. It is listed in every drug interaction reference. And it is missed every day in Caribbean clinical practice -- not because the information is unavailable, but because the mechanism is poorly understood.
Most clinicians know that some drugs "interact." Fewer understand the mechanism. Fewer still can predict, from first principles, which combinations are dangerous and which are not. This report explains the CYP450 system in plain clinical language, identifies the drugs that matter in Caribbean practice, and provides a decision-making framework that can be applied to any new prescription.
1.1 Why CYP450 interactions matter in the Caribbean
The Caribbean patient on multiple medications -- hypertension, diabetes, dyslipidaemia, cardiovascular disease -- is often on at least one drug with a narrow therapeutic index: warfarin, digoxin, lithium, phenytoin, carbamazepine, methotrexate, ciclosporin. These drugs have small margins between a therapeutic dose and a toxic one. When CYP450 inhibitors or inducers are added to the regimen, the plasma level of the narrow-index drug shifts -- sometimes dramatically.
In Caribbean practice, the three most common triggers are:
- Antibiotic prescriptions for acute infections (clarithromycin, erythromycin, ciprofloxacin, metronidazole, fluconazole)
- Herbal teas consumed regularly (St. John's Wort, which is sold in Caribbean health food stores)
- Smoking cessation in patients on clozapine, theophylline, or olanzapine
All three scenarios produce CYP450 interactions that are clinically significant, predictable, and frequently missed.
Section 2: How CYP450 Works -- The Clinical Essentials
2.1 What CYP450 enzymes do
Cytochrome P450 enzymes are a family of proteins in the liver (and, to a lesser extent, the gut wall and other tissues) that metabolise drugs, hormones, and other foreign compounds. Their job is to convert lipid-soluble substances into water-soluble forms that can be excreted in urine or bile.
When a drug is metabolised by a CYP450 enzyme, its plasma level is controlled by how fast the enzyme works. Change the enzyme's activity -- by inhibiting it or inducing it -- and the plasma level of the drug changes.
2.2 The six clinically important enzymes
Of the fifty-plus CYP450 enzymes, six account for the metabolism of the majority of clinically prescribed drugs:
| Enzyme | Notable substrates (drugs it metabolises) |
|---|---|
| CYP1A2 | Clozapine, olanzapine, theophylline, caffeine, warfarin (minor) |
| CYP2C9 | Warfarin (S-enantiomer), phenytoin, glibenclamide, ibuprofen, naproxen, losartan |
| CYP2C19 | Omeprazole, clopidogrel, diazepam, sertraline, amitriptyline |
| CYP2D6 | Tramadol, codeine, metoprolol, haloperidol, risperidone, fluoxetine, paroxetine, tamoxifen |
| CYP3A4 | Amlodipine, simvastatin, atorvastatin, carbamazepine, ciclosporin, erythromycin, clarithromycin, alprazolam, midazolam, quetiapine |
| CYP2E1 | Paracetamol (high doses), ethanol |
2.3 Inhibitors and inducers
An inhibitor blocks the enzyme, causing the substrate drug to accumulate -- higher plasma levels, increased effect, increased toxicity risk. Inhibition is usually rapid, occurring within hours to days of starting the inhibitor.
An inducer increases enzyme production, causing the substrate drug to be cleared faster -- lower plasma levels, reduced effect, potential therapeutic failure. Induction develops more slowly, over days to weeks.
| Category | Effect on substrate | Onset | Clinical consequence |
|---|---|---|---|
| Inhibitor | Substrate level rises | Hours to days | Toxicity from the substrate |
| Inducer | Substrate level falls | Days to weeks | Therapeutic failure of the substrate |
2.4 The prodrug exception
Some drugs are prodrugs -- pharmacologically inactive until metabolised by CYP450 into their active form. For prodrugs, the inhibitor/inducer effect is reversed:
- Inhibitor + prodrug = reduced conversion to active drug = therapeutic failure
- Inducer + prodrug = increased conversion to active drug = excessive active drug = toxicity
Codeine and tramadol are the most clinically relevant prodrugs in Caribbean practice. Both are converted to their active forms by CYP2D6. A CYP2D6 inhibitor (fluoxetine, paroxetine, bupropion) reduces analgesic efficacy. A CYP2D6 ultra-rapid metaboliser (genetic variant, more common in some ethnic populations) produces excessive active metabolite -- respiratory depression from codeine, toxicity from tramadol.
Clopidogrel is another critical prodrug -- converted to its active antiplatelet metabolite by CYP2C19. Omeprazole inhibits CYP2C19. In a patient on clopidogrel for secondary cardiovascular prevention, adding omeprazole potentially reduces antiplatelet efficacy -- a clinically important interaction in a high-risk population.
Section 3: The Major Inhibitors in Caribbean Practice
3.1 CYP3A4 inhibitors
CYP3A4 is the most important enzyme clinically because it metabolises approximately 50% of all prescribed drugs. Inhibiting it produces the broadest range of interactions.
Clarithromycin and erythromycin are the most potent CYP3A4 inhibitors commonly prescribed in the Caribbean. Prescribed for respiratory tract infections, skin infections, and atypical pneumonia, they are added to stable regimens without systematic interaction checking.
Key interactions with CYP3A4 inhibitors (clarithromycin, erythromycin, itraconazole, ketoconazole, grapefruit juice):
| Substrate | Consequence when CYP3A4 is inhibited |
|---|---|
| Warfarin | Increased INR -- bleeding risk |
| Simvastatin / lovastatin | Marked increase in statin levels -- rhabdomyolysis |
| Atorvastatin | Moderate increase -- myopathy (less than simvastatin) |
| Amlodipine | Increased levels -- excessive hypotension |
| Carbamazepine | Increased levels -- toxicity (diplopia, ataxia, drowsiness) |
| Alprazolam / midazolam | Prolonged sedation |
| Quetiapine | Increased levels -- sedation, metabolic effects |
| Ciclosporin | Narrow therapeutic index -- nephrotoxicity, toxicity |
| Fentanyl | Increased opioid effect -- respiratory depression |
Fluconazole is a dual inhibitor of CYP2C9 and CYP3A4. In a diabetic patient on glibenclamide (CYP2C9 substrate), fluconazole prescribed for vulvovaginal candidiasis can cause prolonged, severe hypoglycaemia (Report 8, Section 2.2).
Grapefruit juice inhibits intestinal CYP3A4, not hepatic. Its effect is limited to drugs with significant first-pass intestinal metabolism -- amlodipine (Report 9), simvastatin, atorvastatin. A single glass can increase amlodipine levels by 15-20% and simvastatin levels by up to 330%.
3.2 CYP2C9 inhibitors
| Inhibitor | Relevant substrates | Clinical consequence |
|---|---|---|
| Fluconazole | Warfarin (S), glibenclamide, phenytoin | Bleeding (warfarin); hypoglycaemia (glibenclamide); phenytoin toxicity |
| Metronidazole | Warfarin (S) | Increased INR -- frequent interaction, frequently missed |
| Amiodarone | Warfarin (S), phenytoin | Prolonged INR elevation; phenytoin toxicity |
| Miconazole (oral gel) | Warfarin (S), glibenclamide | Same as fluconazole; often missed as it is perceived as topical (Report 8) |
3.3 CYP2D6 inhibitors
| Inhibitor | Relevant substrates | Clinical consequence |
|---|---|---|
| Fluoxetine | Tamoxifen, tramadol, codeine, metoprolol, haloperidol | Tamoxifen failure; reduced analgesia; metoprolol toxicity (bradycardia) |
| Paroxetine | Same as fluoxetine | Same -- paroxetine is the strongest CYP2D6 inhibitor among SSRIs |
| Bupropion | Tamoxifen, tramadol, codeine | Same |
| Haloperidol | Tramadol, codeine | Reduced prodrug activation |
3.4 CYP1A2 inhibitors
| Inhibitor | Relevant substrates | Clinical consequence |
|---|---|---|
| Ciprofloxacin | Clozapine, olanzapine, theophylline | Clozapine toxicity (seizures, hypotension); theophylline toxicity (arrhythmias) |
| Fluvoxamine | Clozapine, theophylline, warfarin | As above; strongest CYP1A2 inhibitor in clinical use |
Section 4: The Major Inducers in Caribbean Practice
4.1 CYP3A4 and CYP2C9 inducers
Carbamazepine (Report 11) is the most potent clinical inducer. It reduces levels of oral contraceptives, warfarin, quetiapine, haloperidol, tramadol, and doxycycline -- a drug interaction profile so broad that every new prescription for a patient on carbamazepine requires an interaction check.
Rifampicin is the most potent known CYP inducer across multiple enzymes (CYP3A4, CYP2C9, CYP2C19, CYP2D6, CYP1A2). Used in tuberculosis treatment, it reduces plasma levels of warfarin, oral contraceptives, methadone, corticosteroids, and antiretrovirals -- sometimes to sub-therapeutic concentrations.
Phenytoin is a potent inducer of CYP3A4 and CYP2C9 and also auto-induces its own metabolism. It reduces levels of corticosteroids, oral contraceptives, warfarin, and doxycycline.
St. John's Wort (hypericum perforatum) induces CYP3A4 and P-glycoprotein. Available over the counter in Caribbean health food stores. Reduces levels of oral contraceptives, warfarin, ciclosporin, antiretrovirals, and alprazolam. The MHRA has issued formal guidance on St. John's Wort interactions. It is frequently undisclosed -- the three questions from Report 3 apply (Report 11, Section 2.2).
| Inducer | Enzymes induced | Key substrates affected |
|---|---|---|
| Carbamazepine | CYP3A4, CYP2C9, CYP1A2 | OCs, warfarin, quetiapine, haloperidol, tramadol |
| Rifampicin | CYP3A4, CYP2C9, CYP2C19, CYP2D6, CYP1A2 | Warfarin, OCs, methadone, corticosteroids, antiretrovirals |
| Phenytoin | CYP3A4, CYP2C9 | OCs, warfarin, corticosteroids, doxycycline |
| St. John's Wort | CYP3A4, P-gp | OCs, warfarin, ciclosporin, alprazolam, antiretrovirals |
| Smoking (tobacco) | CYP1A2 | Clozapine, olanzapine, theophylline |
4.2 Smoking and CYP1A2 -- a Caribbean-specific concern
Tobacco smoke contains polycyclic aromatic hydrocarbons that induce CYP1A2. Patients who smoke require higher doses of CYP1A2 substrates (clozapine, olanzapine, theophylline) to achieve therapeutic levels.
Smoking cessation is a pharmacokinetic event. When a patient on clozapine or olanzapine stops smoking, CYP1A2 activity decreases over 1-2 weeks. If the antipsychotic dose is not reduced, plasma levels rise by 50-70%, causing toxicity -- sedation, hypotension, seizures.
This interaction is frequently missed because smoking cessation is not seen as a drug interaction. It is. In any patient on a CYP1A2 substrate who stops smoking, the dose should be reviewed at 1-2 weeks.
Section 5: A Decision Framework for New Prescriptions
When adding a new drug to an existing regimen, three questions identify the CYP450 interaction risk:
Question 1: Is the new drug a potent inhibitor or inducer?
High-risk prescriptions requiring immediate interaction assessment: - Clarithromycin, erythromycin (CYP3A4 inhibitors) - Fluconazole, itraconazole (CYP3A4 + CYP2C9 inhibitors) - Metronidazole (CYP2C9 inhibitor) - Ciprofloxacin (CYP1A2 inhibitor) - Fluoxetine, paroxetine (CYP2D6 inhibitors) - Carbamazepine, rifampicin, phenytoin, St. John's Wort (inducers)
Question 2: Is any existing drug a narrow therapeutic index substrate?
High-risk substrates that require monitoring if enzyme activity changes: - Warfarin (CYP2C9, CYP3A4) - Phenytoin (CYP2C9 -- also an inducer) - Ciclosporin (CYP3A4) - Lithium (renal -- not CYP-mediated, but narrow index) - Theophylline (CYP1A2) - Clozapine (CYP1A2) - Digoxin (P-glycoprotein -- not CYP) - Methotrexate (renal/hepatic -- not CYP)
Question 3: Is the new drug or existing drug a prodrug?
If so, the direction of the interaction effect is reversed -- inhibitors reduce efficacy of the prodrug; inducers may increase active metabolite and toxicity.
Key prodrugs in Caribbean practice: codeine (CYP2D6), tramadol (CYP2D6), clopidogrel (CYP2C19), tamoxifen (CYP2D6).
Section 6: Quick-Reference Interaction Table
The most clinically significant CYP450 interactions in Caribbean practice.
| Precipitant drug | Object drug | Enzyme | Effect | Action |
|---|---|---|---|---|
| Clarithromycin | Warfarin | CYP3A4 | INR rises | Monitor INR at 2-3 days; reduce warfarin dose |
| Clarithromycin | Simvastatin | CYP3A4 | Statin rises markedly | Hold simvastatin during antibiotic course |
| Clarithromycin | Carbamazepine | CYP3A4 | Carbamazepine rises | Check level; watch for toxicity |
| Fluconazole | Warfarin | CYP2C9 | INR rises | Monitor INR; expect dose reduction need |
| Fluconazole | Glibenclamide | CYP2C9 | Sulfonylurea rises | Monitor glucose; hypoglycaemia risk |
| Metronidazole | Warfarin | CYP2C9 | INR rises | Monitor INR; common and commonly missed |
| Ciprofloxacin | Theophylline | CYP1A2 | Theophylline rises | Reduce theophylline dose 30-50% |
| Ciprofloxacin | Clozapine | CYP1A2 | Clozapine rises | Monitor; toxicity risk |
| Fluoxetine / paroxetine | Tamoxifen | CYP2D6 | Active metabolite falls | Switch to escitalopram or sertraline |
| Fluoxetine / paroxetine | Tramadol / codeine | CYP2D6 | Active metabolite falls | Analgesic failure |
| Fluoxetine / paroxetine | Metoprolol | CYP2D6 | Metoprolol rises | Bradycardia; reduce dose |
| Carbamazepine | Oral contraceptives | CYP3A4 | OC levels fall | Contraceptive failure; alternative needed |
| Carbamazepine | Warfarin | CYP3A4 | Warfarin falls | INR drops; loss of anticoagulation |
| Carbamazepine | Quetiapine | CYP3A4 | Quetiapine falls | Therapeutic failure |
| Rifampicin | Warfarin | CYP3A4+2C9 | Warfarin falls markedly | Major INR reduction; frequent dose increases |
| Rifampicin | OCs | CYP3A4 | OC fails | Alternative contraception during TB treatment |
| St. John's Wort | Warfarin | CYP3A4 | Warfarin falls | INR drop; thromboembolism risk |
| St. John's Wort | OCs | CYP3A4 | OC fails | Contraceptive failure; commonly undisclosed |
| Smoking | Clozapine | CYP1A2 | Clozapine falls | Higher doses needed; reduce on cessation |
| Smoking | Olanzapine | CYP1A2 | Olanzapine falls | As above |
| Grapefruit juice | Amlodipine | CYP3A4 | Amlodipine rises | Excessive hypotension |
| Grapefruit juice | Simvastatin | CYP3A4 | Simvastatin rises markedly | Myopathy; avoid grapefruit with simvastatin |
| Omeprazole | Clopidogrel | CYP2C19 | Active metabolite falls | Reduced antiplatelet efficacy; consider alternative PPI |
Section 7: About ElesRx
ElesRx identifies CYP450 interactions automatically when a clinician enters a patient's full medication list. The interaction database includes enzyme-level data for substrates, inhibitors, and inducers, allowing the system to flag not only known pairs but also new combinations based on shared enzyme pathways. When a new prescription is added that involves a CYP inhibitor or inducer, ElesRx identifies all narrow-index substrates already on the patient's list and generates a relevant alert.
The tool is available at elesrx.com. ElesRx is a product of PIPPS Smart Apps, a division of J.C. Epiphany Limited (Jamaica, est. 1998).
Section 8: Methodology and References
8.1 Data sources
CYP450 interaction data is drawn from the ElesRx clinical database, verified against DailyMed, the European Medicines Agency, Health Canada, and published pharmacology references. Enzyme classification and clinical significance ratings align with published interaction databases (Lexicomp, Stockley's Drug Interactions -- used for verification only; not reproduced).
8.2 Limitations
CYP450 interactions are one mechanism of drug interaction. Pharmacodynamic interactions (additive effects, antagonism) and transporter-mediated interactions (P-glycoprotein, OATP) are equally important clinically. This report covers CYP450 specifically. Comprehensive interaction checking accounts for all mechanisms.
8.3 Author and conflict of interest disclosure
This report was authored by Juliet Duncan, BPharm, founder of J.C. Epiphany Limited and developer of ElesRx. The author has a commercial interest in ElesRx. This report is published without an access gate as a contribution to Caribbean clinical education. No external funding was received.
8.4 Citation
Duncan J. The CYP450 Report: When One Drug Changes How Another Works. ElesRx Clinical Reports, Report 12. Published 2026 at elesrx.com/reports/cyp450-report/. J.C. Epiphany Limited, Jamaica.
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Flockhart DA. Drug interactions: cytochrome P450 drug interaction table. Indiana University School of Medicine. 2007. Accessed 2026. medicine.iu.edu/pharmacology/cyp450
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Pirmohamed M. Drug-grapefruit juice interactions. BMJ. 2013;346:f1. doi:10.1136/bmj.f1
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Henderson L, Yue QY, Bergquist C, Gerden B, Arlett P. St John's Wort (Hypericum perforatum): drug interactions and clinical outcomes. Br J Clin Pharmacol. 2002;54(4):349-356. doi:10.1046/j.1365-2125.2002.01683.x
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Pharmacogenomics Knowledge Base (PharmGKB). Accessed 2026. pharmgkb.org