Pharmacokinetics describes what the body does to a drug, how it is absorbed, how it distributes through tissues, how it is chemically transformed, and how it is eliminated. Understanding the pharmacokinetics of kratom alkaloids is essential for interpreting research findings, understanding the time course of alkaloid effects, and appreciating the complexity of interactions between mitragynine and its metabolites, including 7-hydroxymitragynine.
The pharmacokinetic data on kratom alkaloids in humans is considerably more limited than the pharmacological data from in vitro and animal studies. This article summarises what is known from published human and animal pharmacokinetic research, with appropriate acknowledgement of the gaps.
The ADME Framework
Pharmacokinetics is conventionally organised around four processes: Absorption, Distribution, Metabolism, and Excretion (ADME). Each stage influences the concentration of active compounds available to interact with target receptors and the duration for which that interaction persists.
Absorption
Absorption refers to the process by which a compound moves from its site of administration into systemic circulation.
Oral Absorption of Mitragynine
The most common route of kratom consumption is oral, either as powdered leaf, encapsulated powder, tea, or more recently as tablet or extract formulations. Following oral ingestion, mitragynine is absorbed through the gastrointestinal mucosa. A pharmacokinetic study conducted in healthy Malaysian volunteers, one of the few human PK studies published for kratom alkaloids, found that mitragynine reached peak plasma concentration (Cmax) at approximately 0.8 to 2 hours post-ingestion, indicating relatively rapid gastrointestinal absorption.
The oral bioavailability of mitragynine, the fraction of an ingested dose that reaches systemic circulation, has been studied in animal models but is not definitively established in humans. Animal studies suggest mitragynine undergoes meaningful first-pass hepatic metabolism, which would reduce the fraction of an oral dose reaching systemic circulation relative to an intravenous dose.
Absorption of 7-Hydroxymitragynine
The pharmacokinetics of 7-hydroxymitragynine as an isolated compound following oral administration are less well characterised in published human studies than those of mitragynine. Animal pharmacokinetic data suggest 7-OH is absorbed orally, with some studies finding rapid absorption and distribution. The pharmacokinetics of 7-OH when consumed as part of a whole-kratom preparation are complicated by its simultaneous generation as a metabolite of mitragynine during hepatic processing.
Distribution
Following absorption into systemic circulation, alkaloids distribute through body compartments. Distribution is influenced by plasma protein binding (compounds bound to plasma proteins are not freely available to cross cell membranes), lipophilicity (fat-soluble compounds distribute more readily into fatty tissues and across cell membranes), and the degree of ionisation at physiological pH.
Mitragynine is highly lipophilic, a characteristic shared by many terpenoid indole alkaloids. This lipophilicity predicts extensive distribution into fatty tissues and across the blood-brain barrier, which is consistent with its CNS activity. Plasma protein binding of mitragynine has been reported in animal studies at approximately 98%, meaning a small free fraction mediates receptor interactions.
Published research using rat brain tissue has confirmed the ability of mitragynine and its metabolites to cross the blood-brain barrier and achieve measurable brain concentrations. The ratio of brain to plasma concentrations over time provides information about CNS penetration and the dynamics of CNS exposure relative to peripheral exposure.
Metabolism
Kratom alkaloid metabolism occurs primarily in the liver through cytochrome P450 (CYP) enzyme systems. This step is particularly significant for understanding the relationship between mitragynine and 7-hydroxymitragynine.
Phase I Metabolism of Mitragynine
In vitro studies using human liver microsomes have identified the specific CYP isoforms responsible for mitragynine metabolism. CYP3A4, the most abundant hepatic CYP enzyme and the primary metaboliser of approximately 50% of all clinically used drugs, has been identified as the principal enzyme responsible for mitragynine's phase I metabolism.
The metabolic products of mitragynine include 7-hydroxymitragynine (via oxidation at the 7-position), as well as other hydroxylated metabolites and demethylated metabolites. This establishes the in vivo metabolic pathway by which mitragynine consumption generates 7-OH in the human body, independent of any 7-OH present in the original preparation.
CYP Inhibition and Drug Interaction Potential
Mitragynine has been found in vitro to inhibit several CYP isoforms, including CYP3A4, CYP2D6, and CYP2C9, at concentrations achievable following typical doses. CYP inhibition is pharmacokinetically significant because it can alter the metabolism of co-administered compounds that are substrates of the same enzyme, potentially increasing or decreasing their plasma concentrations.
This theoretical drug interaction potential has been noted in pharmacological literature as a safety concern requiring further clinical investigation, particularly given that kratom is sometimes used concurrently with other substances. Published case reports and adverse event analyses have documented cases involving polypharmacy, though establishing causation from these reports is methodologically challenging.
Phase II Metabolism and Metabolite Identification
Phase II metabolism involves conjugation reactions that typically increase water solubility and facilitate renal excretion. Glucuronidation of mitragynine and its hydroxylated metabolites has been identified as an important phase II pathway. Published research has characterised a range of mitragynine phase II metabolites in human urine samples, providing the basis for the development of urine-based drug testing methods for kratom exposure.
Excretion
Mitragynine and its metabolites are excreted primarily via the urine. Published human pharmacokinetic data found an elimination half-life for mitragynine of approximately 9 to 24 hours in human subjects, a range that reflects inter-individual variability in metabolism and the dose-dependence of pharmacokinetic parameters.
The relatively long half-life of mitragynine relative to many other psychoactive substances has implications for detection windows in drug testing contexts. Published research on urinary mitragynine metabolite detection has informed the development of immunoassay-based urine drug screens for kratom. Mitragynine pseudoindoxyl, a major urinary metabolite, has been studied as a potential urinary biomarker for kratom exposure given its specificity and detectability window.
Faecal excretion also contributes to elimination; the proportion of an oral dose eliminated via biliary excretion into faeces versus renal excretion into urine has been examined in animal studies but is not definitively characterised in humans.
Key Pharmacokinetic Gaps in the Published Literature
The pharmacokinetic evidence base for kratom alkaloids has important limitations that should be acknowledged:
- Published human PK studies are few in number and limited in sample size
- The contribution of in situ-generated 7-OH (from hepatic mitragynine metabolism) versus pre-formed 7-OH in a preparation to overall 7-OH plasma exposure has not been cleanly established in human studies
- PK parameters in populations with hepatic or renal impairment are not characterised
- The pharmacokinetics of concentrated 7-OH preparations (as distinct from whole-leaf kratom) have not been studied in published human trials
- Dose-concentration relationships have not been characterised across the full range of doses reported in consumer use surveys