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USMLE Step 1Pharmacology: principles and autonomic drugsEP 09 of 16

CYP450 inducers and inhibitors: how they change drug levels

A drug's dose can stay the same while its level changes. Cytochrome P450 inducers and inhibitors turn the clearance dial, and the level, and the effect, follow.

  • 5:16 video
  • 3 min read
  • 15 sources
  • Published 15 Nov 2026
Video lesson · 5:16 · Watch on YouTubeSeries: Pharmacology: Pharmacokinetics & Autonomic Drugs · @medservatory
Chapters (13)
By the end of this lesson

Predict the effect of CYP450 inducers and inhibitors on co-administered drug levels and recognise high-risk interactions.

The short version

  1. At steady state, drug level = dosing rate / clearance, so changing clearance changes the level: double the clearance, halve the level; halve the clearance, double the level.
  2. A CYP450 inhibitor lowers clearance and raises the other drug's level; an inducer raises clearance and lowers it.
  3. Inhibition usually starts with the first doses of the inhibitor; induction needs new protein synthesis, so it takes days to weeks to build up and to wear off.
  4. Classic inducers: rifampin, carbamazepine, phenytoin, phenobarbital, St John's wort. Classic inhibitors: macrolides (not azithromycin), azole antifungals, ritonavir, cimetidine, grapefruit juice.
  5. The danger is greatest for narrow-margin drugs such as warfarin, cyclosporine and tacrolimus, because even a modest change in level can move them out of the safe range.

CYP450 inducers and inhibitors: same dose, different level

A man on stable warfarin starts treatment for tuberculosis. Three weeks later his INR has fallen from 2.6 to 1.1. His warfarin dose never changed, so what did?

The answer is CYP450 inducers and inhibitors. Many drugs leave the body by metabolism, and the main machinery is the cytochrome P450 family, found mostly in the liver and the gut wall.[1][2] When one drug changes how fast that machinery works, it changes the level of another drug.

Steady state
level = dosing rate / clearance

Same dose, different clearance, different level; the drug's effect follows the level. For an oral drug, the dosing rate is the amount that reaches the blood (bioavailability x dose rate), so blocking CYP450 in the gut wall raises the level too.

So clearance is the hidden dial. Double the clearance and the level halves; halve the clearance and the level doubles.[6]

Inhibitors versus inducers: which way the level moves

An inhibitor blocks the enzyme, so clearance falls and the other drug's level rises.[3][2]

An inducer makes the cell build more enzyme, so clearance rises and the other drug's level falls.[2][4]

CYP450 inhibitors and inducers compared
InhibitorInducer
Effect on the enzymeBlockedMore enzyme built
Clearance of the other drugFallsRises
Level of the other drugRisesFalls

Timing: inhibition is fast, induction is slow

Inhibition usually starts with the first doses of the inhibitor, and fades as the inhibitor is cleared. For some inhibitors it lingers until new enzyme is made. The other drug's level then climbs to its new steady state over about five of its own half-lives.[2][4][5]

Induction needs new protein synthesis, so it takes days to weeks to build up, and days to weeks to wear off after the inducer stops.[4][2]

The classic CYP450 inducers and inhibitors

The names to know
Inducers (clearance up, level down)Inhibitors (clearance down, level up)
RifampinMacrolides: clarithromycin, erythromycin
CarbamazepineAzole antifungals, such as ketoconazole
PhenytoinRitonavir
PhenobarbitalCimetidine
St John's wortGrapefruit juice

The inducer names are supported by[2][7]; the inhibitor names by[5][3][2][10].

Worked example: how clearance sets the level

A drug is given at 10 mg per hour, and its clearance is 5 L per hour. The steady-state level is the dosing rate divided by the clearance.[6]

Dosing rate 10 mg/h
  1. No interacting drug: 10 / 5 = 2 mg/L
  2. Inhibitor halves clearance to 2.5 L/h: 10 / 2.5 = 4 mg/L (the level doubles)
  3. Inducer doubles clearance to 10 L/h: 10 / 10 = 1 mg/L (the level halves)

Same dose, different level, and the drug's effect follows the level.

High-risk drug interactions: narrow-margin drugs

That is the warfarin patient. Rifampin induced the enzymes that clear warfarin, so its level and its anticoagulant effect fell.[1][4][10][11]

Rifampin also speeds the breakdown of estrogen, so oral contraceptives can fail.[1][10]

St John's wort has been reported to cause transplant rejection by lowering cyclosporine levels.[7][8][9]

The risk is greatest for narrow-margin drugs, such as warfarin and the transplant drugs cyclosporine and tacrolimus, which these enzymes clear. Even a modest change in level can move them out of the safe range.[12][13][8][2]

Practice question

Original · USMLE style
Your turnQB-MS-S1-PHARM-007-01

A 46-year-old man received a kidney transplant 2 years ago and has been stable on tacrolimus, with a trough concentration of 7 ng/mL at his last visit. Five days ago he started clarithromycin for community-acquired pneumonia. He now reports a tremor and a headache. Serum creatinine has increased from 1.2 mg/dL to 1.9 mg/dL, and the tacrolimus trough concentration is 22 ng/mL.

Which of the following best explains the change in his tacrolimus concentration?
Pick one option. Your answers stay on this device.

Common questions

How do CYP450 inducers and inhibitors affect drug levels?

An inhibitor blocks the enzyme, so clearance falls and the other drug's level rises. An inducer makes the cell build more enzyme, so clearance rises and the level falls.

Why does the drug level change when the dose does not?

At steady state, level = dosing rate / clearance. If clearance changes, the level changes even though the dose is the same.

Which is faster, enzyme inhibition or enzyme induction?

Inhibition usually starts with the first doses of the inhibitor. Induction needs new protein synthesis, so it takes days to weeks to build up and days to weeks to wear off.

Which drugs are the classic CYP450 inducers?

Rifampin, carbamazepine, phenytoin, phenobarbital and St John's wort.

Which drugs are the classic CYP450 inhibitors?

Macrolides such as clarithromycin and erythromycin (but not azithromycin), azole antifungals such as ketoconazole, ritonavir, cimetidine and grapefruit juice.

Transcript

Read the full narration, by chapter659 words

0:00The case: stable warfarin, INR falls, dose unchanged

A man on stable warfarin starts treatment for tuberculosis. Three weeks later, his INR has fallen from 2.6 to 1.1. His warfarin dose never changed. So what did? Today: predict how enzyme inducers and inhibitors

0:14Today's goal, and cytochrome P450 in the liver and gut wall

change another drug's level, and name the high-risk ones. Most drugs leave the body by metabolism, and the main machinery is the cytochrome P450 family, found mostly in the liver and the gut wall. At steady state, the drug level equals the

0:31The tub model: level = dosing rate / clearance

dosing rate divided by clearance. So change clearance, and you change the level: double the clearance, halve the level; halve the clearance, double the level. An inhibitor blocks the enzyme,

0:44Inhibitors raise levels, inducers lower them

so clearance falls and the other drug's level rises. An inducer makes the cell build more enzyme, so clearance rises and the other drug's level falls.

0:56Timing: inhibition is fast, induction is slow

The timing is different. Inhibition usually starts with the first doses of the inhibitor, and fades as the inhibitor is cleared, though for some inhibitors it lingers until new enzyme is made. Induction needs new protein synthesis, so it takes days to weeks to build up, and days to weeks to wear off after the inducer stops.

1:18The classic inducers and inhibitors

The classic inducers are rifampin, carbamazepine, phenytoin, phenobarbital, and St John's wort. The classic inhibitors are macrolides such as clarithromycin and erythromycin, but not azithromycin; azole antifungals such as ketoconazole; ritonavir; cimetidine; and grapefruit juice. Now the arithmetic.

1:41Worked example: 10 mg/h, 5 L/h, 2.5 L/h, 10 L/h

A drug is given at 10 milligrams per hour, and its clearance is five litres per hour. The steady-state level is 10 divided by five: two milligrams per litre. Add an inhibitor that halves the clearance, to 2.5 litres per hour: the level doubles, to four. Add an inducer that doubles the clearance, to 10 litres per hour: the level halves, to one. Same dose, different level, and the drug's effect follows the level. That is our warfarin patient.

2:12The warfarin patient and oral contraceptive failure

Rifampin induced the enzymes that clear warfarin, so its level and its anticoagulant effect fell. Rifampin also speeds the breakdown of estrogen, so oral contraceptives can fail.

2:27St John's wort, cyclosporine and narrow-margin drugs

St John's wort has been reported to cause transplant rejection by lowering cyclosporine levels. The risk is greatest for narrow-margin drugs, such as warfarin and the transplant drugs cyclosporine and tacrolimus, which these enzymes clear, because a big change in level moves them out of the safe range. Your turn.

2:47Your turn: a practice question

A 46-year-old man received a kidney transplant 2 years ago and has been stable on tacrolimus, with a trough level of seven nanograms per millilitre. Five days ago he started clarithromycin for pneumonia. Now he has a tremor and a headache. Creatinine has risen from 1.2 to 1.9 mg/dL, and the tacrolimus trough is 22. Which of the following best explains the change in his tacrolimus level? A, induction of CYP3A4 by clarithromycin, increasing tacrolimus clearance. B, induction of CYP3A4 by clarithromycin, forming a toxic tacrolimus metabolite. C, inhibition of CYP3A4 by clarithromycin, decreasing tacrolimus clearance. D, inhibition of xanthine oxidase by clarithromycin, decreasing tacrolimus clearance. E, reduced tacrolimus clearance because of the rising creatinine.

3:52Pause, then the wrong options one by one

Pause here and decide. A is out: an inducer would raise clearance and lower the level, the opposite of what we see. B is out: induction would lower the parent drug, so it cannot explain a threefold rise. D is out: the enzyme that clears tacrolimus is CYP3A, not xanthine oxidase. E is out: tacrolimus is cleared mainly by CYP3A metabolism, not by the kidney, so the creatinine rise is a result of the high level, not the cause.

4:28The answer: C

So the answer is C. Clarithromycin inhibits CYP3A4, tacrolimus clearance falls, and the level climbs, with tremor and kidney injury as the signs of toxicity. Three things to keep.

4:42Recap pocket card and next lesson

One: level equals dose rate over clearance, so inhibitors raise levels and inducers lower them. Two: inhibition is fast, induction is slow. Three: the danger is a narrow-margin drug, and the key names are rifampin, carbamazepine, phenytoin, phenobarbital, St John's wort, macrolides, azoles and grapefruit juice. Next, the autonomic receptors: a map.

Sources

Numbers in the text, e.g. [1], point here
  1. Chen J, Raymond K. Roles of rifampicin in drug-drug interactions. Ann Clin Microbiol Antimicrob 2006. pmc.ncbi.nlm.nih.gov/articles/PMC1395332/
  2. Pharmacokinetic drug-drug interaction and their implication in clinical management. J Res Med Sci 2013. pmc.ncbi.nlm.nih.gov/articles/PMC3897029/
  3. Role of cytochrome P450 in drug interactions. Nutr Metab 2008. pmc.ncbi.nlm.nih.gov/articles/PMC2584094/
  4. Antiepileptic drug interactions. Curr Neuropharmacol 2010. pmc.ncbi.nlm.nih.gov/articles/PMC3001218/
  5. Inhibition and induction of CYP enzymes in humans: an update. Arch Toxicol 2020. pmc.ncbi.nlm.nih.gov/articles/PMC7603454/
  6. Pharmacodynamic principles and target concentration intervention. Transl Clin Pharmacol 2018. pmc.ncbi.nlm.nih.gov/articles/PMC6989251/
  7. Clinical relevance of St John's wort drug interactions revisited. Br J Pharmacol 2020. pmc.ncbi.nlm.nih.gov/articles/PMC7056460/
  8. Cytochrome P450 enzyme mediated herbal drug interactions (Part 1). EXCLI J 2014. pmc.ncbi.nlm.nih.gov/articles/PMC4463967/
  9. Hypericum perforatum review. Iran J Basic Med Sci 2022. pmc.ncbi.nlm.nih.gov/articles/PMC9526892/
  10. Clinically important pharmacokinetic drug-drug interactions with antibacterial agents. Rev Esp Quimioter 2024. pmc.ncbi.nlm.nih.gov/articles/PMC11231487/
  11. Real-world evaluation of the interaction between rifampicin and warfarin: a retrospective observational study. Adv Pharmacol Pharm Sci 2025. pmc.ncbi.nlm.nih.gov/articles/PMC12324922/
  12. Tacrolimus pharmacokinetics, pharmacodynamics and pharmacogenetics in kidney transplantation. Curr Drug Metab 2018. pmc.ncbi.nlm.nih.gov/articles/PMC6182932/
  13. Drug-drug interactions between oral anticoagulants and calcineurin inhibitors in nephrotic syndrome (narrow therapeutic index of cyclosporine and tacrolimus). Biomedicines 2026. pmc.ncbi.nlm.nih.gov/articles/PMC13509657/
  14. Review of two immunosuppressants: tacrolimus and cyclosporine (2023). pmc.ncbi.nlm.nih.gov/articles/PMC10761313/
  15. Immunosuppressive neurotoxicities. Int J Mol Sci 2019. pmc.ncbi.nlm.nih.gov/articles/PMC6651704/

The INR values, the 10 mg/h worked example and the practice-question values are invented teaching numbers, not patient data. The level graphs are schematic.

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, resized): tablet, test tube, liver, liver lobule, intestine, enzyme (two versions), capsule, kidney. The tub-and-drain diagrams, graphs, bars, calendar, blister pack, plant and fruit doodles, cards and all handwriting are drawn in code (original). No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-PHARM-007.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 08Loading dose and maintenance dose, explained
  1. EP 01Potency vs efficacy: EC50, Emax and agonist types5:17
  2. EP 02Competitive vs noncompetitive antagonists: effect on EC50 and Emax5:01
  3. EP 03Therapeutic index explained: TD50 / ED505:01
  4. EP 04First-pass metabolism and bioavailability: calculating oral F from AUC5:18
  5. EP 05Volume of distribution: why Vd can be bigger than the body5:17
  6. EP 06Clearance and half-life: t½ = 0.693 × Vd / CL5:01
  7. EP 07Zero-order vs first-order elimination explained5:12
  8. EP 08Loading dose and maintenance dose, explained5:02
  9. EP 09CYP450 inducers and inhibitors: how they change drug levels5:16
  10. EP 10Autonomic receptors: alpha, beta and muscarinic5:33
  11. EP 11Beta blocker selectivity: beta-1, ISA and alpha blockade5:24
  12. EP 12Vasopressors compared: receptors, pressure and heart rate5:46
  13. EP 13Organophosphate poisoning: muscarinic vs nicotinic signs5:36
  14. EP 14Receptor desensitization, downregulation and upregulation5:01
  15. EP 15Beta-lactams and vancomycin: enzyme versus substrate5:04
  16. EP 16Protein synthesis inhibitors: 30S vs 50S antibiotics5:07