This is a deeper, more academic reference. If you’re in active withdrawal, this is not the page for you, head to Withdrawal Help instead. This page is for people trying to understand why kratom feels the way it does, why withdrawal is harder than its raw opioid story suggests, and why some of the lesser-known leaf alkaloids matter.
The short version: mitragynine is the headline alkaloid, but it’s not the whole story. Kratom’s leaf contains 40+ alkaloids; the six “minor” alkaloids below are the better-studied ones beyond mitragynine, and they hit three different receptor systems — µ-opioid, serotonergic (5-HT₁ₐ / 5-HT₂ᵦ), and adrenergic (α-1 / α-2). The serotonergic and adrenergic activity is part of why withdrawal off concentrated kratom products doesn’t fully respond to a pure-opioid medication like Suboxone, see Why Suboxone Might Not Be Working.
The seven alkaloids at a glance
| Alkaloid | Note |
|---|---|
| Mitragynine | The headline alkaloid. Reference. See Kratom Leaf for the lay overview and the leaf’s alkaloid-content profile. |
| Paynantheine | Usually the second-most abundant alkaloid in the leaf. |
| Speciogynine | C-20 diastereomer of mitragynine. |
| Speciociliatine | C-3 diastereomer of mitragynine. |
| Corynantheidine | The kratom µ-opioid antagonist. |
| 9-Hydroxycorynantheidine | The 9-OH analog of corynantheidine. |
| Mitraphylline | An oxindole alkaloid. Poorly characterized at all three receptor systems; included here for structural contrast. |
Chemical structures for each are on the Chemical Structures appendix.
µ-opioid receptor activity (antagonist → agonist spectrum)
Activity at the µ-opioid receptor ranges across these alkaloids from full antagonism (blocks the receptor) to partial agonism (activates it weakly).
| Alkaloid | Activity | Notes |
|---|---|---|
| Corynantheidine | Functional antagonist | Blocks the µ-receptor outright. |
| Paynantheine | Weak antagonist | Competitive, low potency. |
| Speciogynine | Weak antagonist | Competitive, low potency. |
| Mitragynine | Partial agonist | Low efficacy. Kᵢ ≈ 160–230 nM. |
| 9-Hydroxycorynantheidine | Partial agonist | The 9-OH group restores agonism that corynantheidine lacks. |
| Speciociliatine | Partial agonist | Higher affinity than mitragynine. Kᵢ ≈ 50 nM. |
| Mitraphylline | Not reliably characterized | Omitted from ranking. |
Speciociliatine’s agonist-vs-antagonist classification differs between studies; ranking reflects functional behaviour in the more recent receptor-binding work, not abundance in the leaf. The literature disagrees on this one, see León et al., J Med Chem 2021 and Obeng et al., J Med Chem 2020 for the underlying receptor pharmacology.
Activity across three receptor systems
This is the part that matters for withdrawal experience. Kratom’s effects (and the symptoms when you come off it) are the summed action of many alkaloids at many targets, not just the µ-opioid story.
| Alkaloid | µ-opioid | Serotonergic (5-HT) | Adrenergic (α) |
|---|---|---|---|
| Mitragynine | Partial agonist (low efficacy) | Low 5-HT₁ₐ affinity | Weak (micromolar) across α-1 & α-2 |
| Paynantheine | Weak antagonist | High 5-HT₁ₐ, mediates analgesia | Weak / not established |
| Speciogynine | Weak antagonist | High 5-HT₁ₐ, mediates analgesia | Moderate, α-2 |
| Speciociliatine | Partial agonist (higher affinity) | Low | Weak α-2 binding |
| Corynantheidine | Antagonist | Modest | High, α-1D-selective |
| 9-Hydroxycorynantheidine | Partial agonist | (not well characterized) | Weak / not established |
| Mitraphylline | Not reliably characterized | (not well characterized) | Not established |
A few things to read off that table:
- Mitragynine and speciociliatine lean opioid. The “feels like an opioid” part of kratom is mostly these two and 7-OH (the oxidized metabolite of mitragynine, covered separately at 7-OH).
- Paynantheine and speciogynine add a serotonergic layer through 5-HT₁ₐ. This is part of why kratom withdrawal produces some symptoms that resemble SSRI/SNRI discontinuation, brain zaps, wired-but-exhausted feeling, even though the mechanism is different. See Depression and Anhedonia for the recovery side of that picture.
- Corynantheidine is an opioid antagonist with strong adrenergic activity. The α-1D-selectivity is unusual and is the reason corynantheidine sometimes gets discussed as a stimulant-leaning modulator of the kratom effect profile.
- Mitraphylline is included for completeness: it’s an oxindole alkaloid, structurally interesting, but its activity at all three systems is poorly characterized in the published literature.
Why this matters for withdrawal off concentrated products
Concentrated 7-OH tablets and related synthetics deliver primarily the µ-opioid hit at much higher receptor occupancy than the leaf — but daily use of leaf-derived products over time still produces some serotonergic and adrenergic adaptation from the minor alkaloids. When you stop, the µ-opioid side rebalances on the familiar opioid-withdrawal arc; the serotonergic and adrenergic side has its own (overlapping but not identical) recovery curve, which is part of why:
- Withdrawal symptoms include a wired-anxiety, brain-zap quality that pure-opioid models don’t predict
- Suboxone (a pure µ-opioid intervention) covers the opioid receptor side but doesn’t lift the serotonergic/adrenergic side, see Why Suboxone Might Not Be Working
- Adjuncts like clonidine (α-2 agonist, see Helper Medications) target the adrenergic component specifically
The clinical takeaway is on the recovery pages; the pharmacology behind it is on this one.
Reference, not advice. Activity ratings synthesize multiple in vitro and rodent studies. The literature disagrees on some entries (notably speciociliatine at the µ-receptor); the tables above describe the dominant findings, with disagreements flagged in line. Nothing here should be taken as guidance for dosing, identifying products, or self-treatment.
Sources
- León et al., J Med Chem 2021 , minor-alkaloid receptor pharmacology (paynantheine, speciogynine, corynantheidine)
- Obeng et al., J Med Chem 2020 , minor-alkaloid receptor profile
- Chemical Structures (appendix) , 2D skeletal-formula diagrams for each alkaloid above
Where to read next
- Morphine vs. Mitragyna Alkaloids , how structural modifications to mitragynine produce 7-OH, MGM-15, and the other synthetics; affinity comparison against morphine
- Chemical Structures (Appendix) , visual structures for the seven alkaloids above and the modified derivatives
- Compounds: clinical / lay overviews of each named compound (7-OH, MGM-15 — which also covers what’s sold as MIT-A and DHM — pseudo, cat’s claw, kratom leaf)