Medications That Cause Phenoconversion: The Common Enzyme Inhibitors

Not every drug interaction is about two medications fighting for the same target. Many of the most important ones happen because one drug blocks a metabolizing enzyme, changing how your body handles everything else that enzyme processes. That is phenoconversion. The FDA maintains a reference table that sorts these enzyme inhibitors into strong, moderate, and weak, and clinical guidelines translate that strength into a metabolism shift: a strong inhibitor drives the effective activity to zero, a moderate inhibitor roughly halves it, and a weak inhibitor is noted but usually does not move you into a new phenotype on its own.^[1][2] Below are the common offenders grouped by the enzyme they affect. If you take any of these, it is worth knowing what else in your regimen runs through the same enzyme.

CYP2D6 handles many antidepressants and antipsychotics, tamoxifen, and the opioids codeine and tramadol,^[3] so blocking it has wide reach. Several of the strongest inhibitors are themselves common antidepressants.

• Strong: bupropion, fluoxetine, paroxetine, quinidine, terbinafine
• Moderate: abiraterone, cinacalcet, duloxetine, lorcaserin, mirabegron, rolapitant
• Weak: amiodarone, celecoxib, cimetidine, escitalopram, fluvoxamine, labetalol, sertraline

In practice, if you take a CYP2D6 drug and start fluoxetine, paroxetine, or bupropion, your effective CYP2D6 activity can fall to that of a poor metabolizer while you are taking it.

CYP2C19 affects several SSRIs, the antiplatelet clopidogrel,^[4] and proton pump inhibitors. Phenoconversion here is especially consequential for clopidogrel, where reduced metabolism means weaker protection against clots.

• Strong: fluconazole, fluoxetine, fluvoxamine, ticlopidine
• Moderate: cenobamate, felbamate, voriconazole
• Weak: omeprazole

Note that omeprazole, itself a CYP2C19 substrate, is also a weak inhibitor, which is part of why clopidogrel and omeprazole are a frequently flagged pair.

CYP3A4 is involved in metabolizing a very large share of all prescription drugs, so an inhibitor here can raise the levels of several medications at once. This is also the enzyme grapefruit juice acts on.

• Strong: clarithromycin, itraconazole, ketoconazole, posaconazole, ritonavir, cobicistat, nefazodone, voriconazole
• Moderate: aprepitant, ciprofloxacin, diltiazem, dronedarone, erythromycin, fluconazole, grapefruit juice, verapamil
• Weak: amiodarone, cimetidine, cyclosporine, ranolazine, ticagrelor

Because CYP3A4 substrates are so numerous, this is the enzyme where a single new prescription, or a glass of grapefruit juice, is most likely to affect something else you take.

CYP2C9 drives the metabolism of warfarin, phenytoin, and several NSAIDs, all drugs where small changes in level matter.

• Moderate: amiodarone, fluconazole, miconazole
• Weak: disulfiram, fluvastatin, fluvoxamine

Fluconazole is a recurring name across CYP2C9, CYP2C19, and CYP3A4, which is why this single antifungal turns up in so many interaction warnings.

Inhibitors slow enzymes down, but inducers speed them up, which can push a normal metabolizer toward rapid and drop drug levels below where they work. The common strong inducers include rifampin, carbamazepine, phenytoin, St. John's wort, enzalutamide, and apalutamide. Moderate inducers include bosentan, efavirenz, and phenobarbital. Induction builds over a week or two and fades over a similar window, so its effects lag behind starting or stopping the drug.

Whether one of these drugs actually changes your phenotype depends on where you started. The inhibitor strength is applied to your genotype-based activity score, so the same drug can affect two people differently.

This is why a list of inhibitors alone is not enough to act on. The same drug that does nothing to one person's phenotype can convert another's, depending on the genotype underneath. Pairing your genotype-based phenotype from a Gene2Rx report with your real medication list is what makes this reference table specific to you.^[6]

If you recognize one of your own medications in the lists above, the useful next step is to find out your baseline metabolizer status, because the impact of an inhibitor depends entirely on where you started. Testing is especially worth it if you take a CYP2D6, CYP2C19, CYP3A4, or CYP2C9 drug alongside one of the inhibitors named here, or if a medication has felt stronger or weaker than expected since you added something new.

• Can Two Safe Medications Be Dangerous Together? How Metabolism Interactions Work
• What Is Phenoconversion? When Your Medications Override Your Genes
• 23andMe Pharmacogenetics: How to Get a Drug Response Report From Your Existing Data
• AncestryDNA for Drug Testing: Get Pharmacogenetics From Your Ancestry Data
• Looking for a GeneSight Alternative? Here's the Short Answer
• MyHeritage Pharmacogenetics: Use Your MyHeritage DNA for a Drug Response Report

On CYP2D6, the standouts are fluoxetine, paroxetine, and bupropion, all classed as strong inhibitors. Fluvoxamine is a strong CYP2C19 inhibitor and a weak CYP2D6 inhibitor. This matters because these are widely prescribed, and a person taking one can be functionally converted to a poor metabolizer for any drug that runs through the affected enzyme.

Yes. Grapefruit juice is classed as a moderate CYP3A4 inhibitor, the same category as diltiazem or erythromycin, so it is handled exactly like an inhibitor drug when working out interactions. We cover it in detail in our grapefruit juice guide.

Some do. Cimetidine, an over-the-counter acid reducer, is a weak inhibitor of several enzymes. St. John's wort, a herbal supplement, is a strong CYP3A4 inducer. Because they are not prescriptions, they are easy to leave off a medication list, which is exactly why a complete list, including supplements, matters.

It depends on your genotype and on the inhibitor's strength, so a list alone cannot tell you. The reliable way is to combine your genotype-based phenotype with your current medications and recompute the effective phenotype. The Gene2Rx app does this automatically as you add or remove drugs.