CYP2D6 and CYP3A4: The Principle Behind the Interaction Case Reports
In the literature on Mitragyna speciosa, case reports turn up on combinations with very different medicines — with substances from groups that have nothing to do with one another pharmacologically. At first glance that looks like many different problems. In fact there is a single, well understood principle behind it, one that applies in exactly the same way to a large part of all medicines and to a few entirely everyday foods.
First the classification: interactions through CYP are the normal case
One sentence belongs at the beginning, and it is almost always missing from the discussion: interactions through cytochrome P450 enzymes are not an exotic special case but the everyday reality of drug therapy.
The best known example sits in many kitchens. Grapefruit juice inhibits CYP3A4 in the intestinal wall and thereby raises the blood levels of a whole series of prescribed medicines; a review in the Canadian Medical Association Journal (volume 185, issue 4, pages 309–316) lists dozens of affected active ingredients and describes how a single glass can already be enough. Review of grapefruit interactions The counterpart is supplied by St John's wort, a freely available botanical preparation: a controlled study in the Journal of the American Medical Association (2003, volume 290, issue 11, page 1500) showed that it induces CYP3A4, that is, increases the amount of enzyme and accelerates the breakdown of other substances. Study on St John's wort
Added to this are countless interactions of medicines with one another. Anyone speaking about CYP interactions is therefore not speaking about a peculiarity of this one plant but about a principle that applies to citrus fruits, to teas from the drugstore and to half the medicine cabinet. That does not make the topic unimportant — it makes it general, and therefore learnable.
What cytochrome P450 enzymes are
The body has to get rid of substances that do not originate from it. Many of them are fat-soluble and can hardly be excreted through the kidney. The liver therefore converts them: it attaches oxygen to the molecule, making it more water-soluble and thus excretable. The enzyme family that takes on this first conversion step is called cytochrome P450, abbreviated CYP — named after its characteristic absorption peak at 450 nanometers.
The individual members carry identifiers made up of a number, a letter and a number: CYP3A4, CYP2D6, CYP2C9 and so on. They sit chiefly in the liver cells, but to a considerable extent also in the intestinal wall — a detail that becomes important further below.
CYP2D6 and CYP3A4: two enzymes, a large share of all medicines
Two representatives of this family dominate drug metabolism. CYP3A4 is involved in the breakdown of roughly half of the medicines used clinically. CYP2D6 follows with around a fifth to a quarter — remarkable, because this enzyme makes up only a few percent of the liver's cytochrome content.
From this follows a simple piece of arithmetic: anyone who influences one of these enzymes potentially influences a great many medicines at once — not because they are related to one another, but because they share the same route of breakdown.
Inhibition and induction: the image of the blocked exit ramp
Two directions are possible. In inhibition, one substance blocks the enzyme, and another substance that needs this enzyme for its breakdown builds up in the blood. The image of the blocked exit ramp carries well here: the motorway is just as busy as before, but the exit is closed, and the tailback grows. That is the grapefruit case. In induction the opposite happens: the cell forms more enzyme, the exit becomes wider, and the concentration of the affected substance falls. That is the St John's wort case.
A third distinction is decisive for what follows. Reversible inhibition ends as soon as the inhibiting substance has disappeared. Mechanism-based inhibition, also called time-dependent, by contrast puts the enzyme permanently out of action; activity only returns once the cell has formed new enzyme. It therefore has a longer after-effect than the presence of the inhibiting substance would suggest.
What has been measured for mitragynine
The following findings come from cell systems and isolated enzyme preparations — from human liver microsomes, intestinal microsomes and recombinantly produced individual CYP isoforms. An isolated enzyme system answers the question "is it possible in principle?", not the question "does it happen in humans?". A 2021 review in Frontiers in Pharmacology brings together the state of knowledge (Hanapi, Chear, Azizi and Yusof, doi 10.3389/fphar.2021.751656). Three points from it:
- CYP3A4 is the main route of breakdown. The work records that CYP3A4 is the most important isoform for the breakdown of mitragynine, with a small or negligible contribution from CYP2C9, CYP2C19 and CYP2D6.
- Among the isoforms examined, 7-hydroxymitragynine arose exclusively via CYP3A4. The same enzyme that breaks mitragynine down therefore produces a further alkaloid in the process.
- Mitragynine inhibits CYP2D6 in the test system. The collated values of the inhibition constant Ki lie between about 1.1 and 13 micromoles per liter: 12.86 µM in a 2013 paper (non-competitive), 1.1 µM and 1.17 µM in two papers from 2020 and 2021 (competitive in each case).
For CYP3A4 the same compilation describes a time-dependent, mechanism-based inhibition. The IC50 value — the concentration at which enzyme activity falls to half — dropped in the time-dependent experimental setup to 2.6 µM in liver microsomes and 3.2 µM in intestinal microsomes. For CYP2D6 no such time dependence was observed. Review 2021
The limited conversion as a built-in ceiling
That CYP3A first produces the more potent 7-hydroxymitragynine is often mentioned only as an analytical complication. The more interesting part of this finding has been measured in an animal model.
Hill and colleagues published a paper in the British Journal of Pharmacology in 2022 (volume 179, issue 14, pages 3875–3885) whose title already contains the statement: the respiratory-depressant effects of mitragynine are limited by its conversion to 7-OH-mitragynine. What was examined was breathing in awake, freely moving mice by whole-body plethysmography, with all substances given orally. The authors' findings:
- The respiratory-depressant effect of mitragynine showed a ceiling effect: doses above 10 mg per kilogram produced the same effect as 10 mg per kilogram — more substance did not lead to more effect.
- Directly administered 7-OH-mitragynine, by contrast, behaved dose-dependently, that is, without this ceiling.
- Inhibition of CYP3A by ketoconazole reduced both the respiratory depression and the antinociceptive effect of mitragynine — the effects of 7-OH-mitragynine remained untouched by it.
From this the authors conclude that the limited rate of conversion of mitragynine into its active metabolite produces a built-in ceiling effect, and they write that such "metabolic saturation" at high doses could underlie a more favorable safety profile. Study 2022
This is a finding from an animal model, and it remains one. But it shows that the same fact — potency hangs on a limited conversion step — has two readings: analytically it is a complication, because one cannot measure only the parent substance in the blood. Pharmacologically, in the animal experiment, it is an upper limit. The two stand side by side, and the second is regularly suppressed in summaries.
The CYP2D6 polymorphism: why people break substances down at different speeds
CYP2D6 is the most variable enzyme of the family: the same gene occurs in the population in many forms, and a 2025 review names more than 100 known allele variants. The usual approach is a division into four classes.
| Class | Genetic basis | Speed of breakdown |
|---|---|---|
| poor metabolizers | two non-functional alleles | strongly reduced |
| intermediate metabolizers | weakened allele combination | reduced |
| normal metabolizers | at least one fully functional allele | reference range |
| ultrarapid metabolizers | additional functional gene copies | increased |
The proportion of poor metabolizers is given as about 5 to 10 percent for European populations; the distribution differs considerably between population groups. Review 2025
This, incidentally, is one of the reasons why case reports are so hard to generalize here: two people with identical exposure and identical concomitant medication can differ several-fold in their speed of breakdown.
One mechanism, many manifestations
With this the initial observation can be placed. What connects the medicines named in the case reports is not their pharmacological kinship but their route of breakdown: a large proportion of them are metabolized via CYP2D6 or CYP3A4. It would then not be eight different mechanisms but one with eight manifestations — the more economical explanation, and at the same time a hypothesis that can be tested. That is exactly what was done.
What the clinical study found
In 2023 the first clinical study on this question appeared in Clinical Pharmacology and Therapeutics (Tanna and colleagues, volume 113, issue 6, pages 1315–1325, doi 10.1002/cpt.2891). Twelve healthy adults received a single tea preparation made from 2 grams of leaf material, together with two probe substrates — measuring substances whose route of breakdown is precisely known and from whose blood concentration one can read how active the responsible enzyme currently is. Used were 2.5 mg midazolam as a probe for CYP3A and 30 mg dextromethorphan as a probe for CYP2D6. The result turned out considerably more restrained than the cell experiments would have led one to expect.
| Probe substrate | Enzyme measured | Ratio of total exposure (90 percent interval) |
|---|---|---|
| Dextromethorphan | CYP2D6 | 0.99 (0.83–1.19) |
| Midazolam | CYP3A | 1.39 (1.23–1.57) |
For the CYP2D6 probe no effect was measurable — the value 0.99 lies practically exactly on the line marked "no change", and this although mitragynine clearly inhibits this enzyme in the isolated system. For the CYP3A probe there was an increase by a factor of 1.39. For context: the US Food and Drug Administration classifies a substance as a moderate inhibitor only from a doubling of exposure onward, and as a strong inhibitor from a fivefold increase. Classification by the US Food and Drug Administration The measured value does not reach this threshold.
An accompanying modeling calculation attributed the increase chiefly to a time-dependent inhibition of CYP3A in the intestinal wall — the place where an orally ingested substance meets the highest concentration. It is the same site and the same mechanism through which grapefruit juice also acts. Clinical study 2023
Why specialists consider the mechanisms more varied than is often presented
The mechanism question also has a regulatory side, and there it has been contradicted. For a new edition of their assessment under the eight factors of the US Controlled Substances Act, Henningfield, Wang and Huestis evaluated more than 100 papers published since 2018 (Frontiers in Pharmacology 2022). They arrive at the conclusion that scheduling under this law is not to be recommended, and argue instead for a regulation that prevents contaminated, adulterated and improperly marketed products. Their first analysis of 2018 in Psychopharmacology (volume 235, issue 2, pages 573–589) had recorded that the abuse potential of mitragynine lies within the range of many uncontrolled substances. Eight-factor analysis 2021 Eight-factor analysis 2018
Honesty requires the disclosure the authors themselves make: through their company they advise, among others, a US trade association on this topic. The disclosure appears in both papers; the papers themselves are peer-reviewed and verifiable. For the mechanism question of this article, one point matters in that: a substance whose potency hangs on a limited conversion step and whose points of attack cannot be reduced to a single receptor system is not described by a one-word category.
The limits: in vitro is not in vivo
The discrepancy between test tube and human being is the instructive point of this topic, and the reasons for it are named in the papers themselves.
- Concentration. In a cell experiment a concentration is set; in the body it establishes itself. According to the assessment of the 2025 review, the freely available concentration from 2 grams of leaf material was too low to inhibit CYP2D6 measurably.
- Single dose. What was tested was a single administration. A mechanism-based inhibition only builds up through repeated intake.
- Twelve healthy adults. The group says nothing about people with liver disease, a divergent CYP2D6 genotype or long-term medication.
- One product, one amount, one route of intake. The alkaloid content of plant starting material varies considerably between origin, harvest and processing; other preparations are not covered.
What is established and what is being worked on
Established: CYP3A4 is the main route of breakdown of mitragynine and produces 7-hydroxymitragynine in the process. In isolated systems CYP3A4 is inhibited time-dependently, CYP2D6 competitively. In the only clinical probe study so far, no measurable effect was seen for CYP2D6 and an increase by a factor of 1.39 for CYP3A, which does not reach the threshold for a moderate inhibitor and was attributed by calculation chiefly to the intestinal wall. In the animal model the rate of conversion limits the respiratory-depressant effect of mitragynine from above.
Open and being worked on: the behavior on repeated intake over several days, with other preparations and amounts, with a divergent CYP2D6 genotype and with existing long-term medication. From a mechanism one can derive what to look for — not what happens in an individual case.
In practical terms the same applies here as with grapefruit juice and St John's wort: questions about interactions with prescribed medicines belong in the hands of the treating physician or the pharmacy. That is where the information that matters is held — the complete medication, the medical history and, where applicable, the genotype.
Further reading
- What is mitragynine?
- What is 7-hydroxymitragynine?
- Pharmacology light: How the body processes plant compounds
- Receptors explained simply with kratom context
Legal notice
This article is for informational purposes only and does not constitute legal advice. Its content is not intended to encourage consumption. Laws may change; the applicable regulations and information from official bodies are authoritative. Image source: https://www.kratoein.com/