This is a deeper, more academic reference. If you’re in active withdrawal, this is not the page for you — head to #sos-resources or #what-to-expect 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 ones below are six of the better-studied “minor” alkaloids, charted against mitragynine across three receptor systems — µ-opioid, serotonergic, and adrenergic. 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 #suboxone-isnt-working).
Kratom’s Minor Alkaloids: Serotonergic & Adrenergic Activity Beyond the µ-Opioid Receptor
Mitragynine dominates kratom, but the leaf carries dozens of other alkaloids. This chart compares six of the better-studied minor alkaloids against mitragynine across three receptor systems — the µ-opioid receptor, serotonin (5-HT) receptors, and adrenergic (α) receptors — the targets behind kratom’s effects on pain, mood, alertness, and opioid-withdrawal relief.
Kratom’s effects are the summed action of many alkaloids at many targets. Mitragynine and speciociliatine lean opioid; paynantheine and speciogynine add a serotonergic layer; corynantheidine is an opioid antagonist with strong adrenergic activity. Solid chips = well-characterised activity; faded chips = weak, modest, or not yet established. Mitraphylline (an oxindole) is poorly characterised at all three systems and is shown only for structural contrast.
Speciociliatine’s agonist-vs-antagonist classification differs between studies. Mitraphylline is omitted — its opioid activity is not reliably characterised. Position reflects functional behaviour, not abundance in the leaf.
Principal indole alkaloid — corynanthe type
Indole alkaloid — vinyl analog of mitragynine
Indole alkaloid — C-20 diastereomer of mitragynine
Indole alkaloid — C-3 diastereomer of mitragynine
Indole alkaloid — mitragynine without the 9-methoxy group
Indole alkaloid — 9-hydroxy analog of corynantheidine
Pentacyclic oxindole alkaloid — a distinct ring system
- By far the largest single alkaloid — roughly 40–66% of total alkaloid content
- The benchmark against which the others are compared
- Open methoxyacrylate “ring E”; indole core; tertiary amine
- Typically the 2nd most abundant alkaloid — about 8–9%
- Differs from mitragynine by a vinyl (C=C) group in place of the ethyl side chain
- Same indole core and methoxyacrylate as mitragynine
- Often the 3rd most abundant — around 6–7%
- Same 2D structure as mitragynine; epimeric at C-20 — the 3D shape differs
- Stereochemistry, not connectivity, sets it apart
- Only ~1% of leaf alkaloid content …
- … but a major circulating alkaloid in humans — well absorbed, slowly cleared
- C-3 epimer of mitragynine — again a stereochemical difference
- A genuinely minor alkaloid — usually well under 1%
- Structurally mitragynine minus the aromatic 9-methoxy group
- Loss of that one substituent changes its functional behaviour markedly
- A minor alkaloid present in small amounts
- Corynantheidine with a phenolic –OH restored at the 9-position
- Useful for showing how the C-9 substituent steers activity
- A minor alkaloid; an oxindole, not an indole
- The indole nitrogen ring is rearranged to a spiro-oxindole
- Also a major alkaloid of cat’s claw (Uncaria tomentosa)
- Partial µ-opioid agonist with low efficacy (Eₘₐₓ ≈ 34%)
- Binding affinity Kᵢ ≈ 160–230 nM at human µ-receptor
- Carries the main opioid-like signal of the whole plant
- Low-potency competitive antagonist at the µ-receptor
- Weak µ binding; little activity at κ- or δ-receptors
- Does not itself produce opioid-type agonism
- Weak µ-receptor affinity
- Behaves as a low-potency competitive µ antagonist in functional assays
- Negligible κ / δ activity
- Higher µ affinity than mitragynine — Kᵢ ≈ 50 nM (~3× stronger binding)
- Generally reported as a partial µ-agonist
- Agonist-vs-antagonist reports conflict between studies
- A functional µ-opioid antagonist
- Can reverse the effects of morphine and of mitragynine
- Binding affinity Kᵢ ≈ 120 nM at the µ-receptor
- A partial µ-agonist
- Restoring the 9-OH flips behaviour toward agonism (vs antagonist corynantheidine)
- Illustrates how the C-9 position governs agonist / antagonist character
- Opioid activity is poorly characterised
- Older reports of opioid-receptor binding are weak and unconfirmed
- No reliable agonist / antagonist classification
- Low affinity at 5-HT₁ₐ (Kᵢ ≈ 5.9 µM)
- Weak reported interactions at 5-HT₂ꞯ and 5-HT₇
- Serotonin receptors are a minor target for mitragynine itself
- High affinity at 5-HT₁ₐ and 5-HT₂ᵦ receptors
- Produces 5-HT₁ₐ-mediated antinociception that is opioid-independent
- Binds 5-HT₂ᵦ but does not activate it
- High affinity at 5-HT₁ₐ and 5-HT₂ᵦ — like paynantheine
- Produces 5-HT₁ₐ-mediated antinociception and lower-lip retraction
- Does not activate 5-HT₂ᵦ
- Low serotonin-receptor binding
- Much weaker at 5-HT₁ₐ / 5-HT₂ᵦ than paynantheine or speciogynine
- Its C-3 configuration favours opioid over serotonergic targets
- Modest serotonin-receptor binding
- Not its principal target — adrenergic and opioid effects dominate
- Serotonergic contribution likely small
- Activity at 5-HT receptors is not well characterised
- Few or no dedicated serotonin-binding studies
- Profile remains an open question
- Serotonin-receptor activity is not well characterised
- Oxindole alkaloids have been studied mostly at non-CNS targets
- No reliable 5-HT data
- Moderate, non-selective binding across α-1 (A/B/D) and α-2 (A/B/C)
- Affinities in the low-micromolar range (Kᵢ ≈ 1.3–9.3 µM)
- Its antinociception is partly reversed by α-2 antagonists
- Adrenergic binding is less thoroughly characterised
- Some interaction reported but not systematically mapped
- Serotonergic activity is the better-documented feature
- Non-selective α-2A / 2B / 2C binding (Kᵢ ≈ 0.36–2.6 µM)
- Comparable to mitragynine at the α-2 receptors
- The S-configuration at C-3 is required for this α-2 binding
- Weak adrenergic binding (Kᵢ > 10 µM at α-2)
- The C-3 epimer largely loses α-2 affinity
- Confirms C-3 stereochemistry controls adrenergic binding
- High, selective affinity at the α-1D receptor (Kᵢ ≈ 42 nM)
- In the same affinity class as α-1 blocker drugs
- Little binding at α-2 receptors
- Adrenergic profile is not well characterised
- No systematic α-receptor binding data
- Behaviour at α-1 / α-2 remains unknown
- Adrenergic activity is not well characterised
- No reliable α-receptor binding data
- Pharmacology studied mainly outside the CNS
* Activity levels are qualitative summaries compiled from peer-reviewed in vitro binding / functional assays and in vivo rodent studies; potency and even agonist-vs-antagonist classification vary with the receptor model, species, and assay used. † Kᵢ (inhibition constant) values are approximate — a lower Kᵢ means tighter binding. ‡ “Not established” means no reliable published data, not absence of activity.
Beyond opioids — kratom’s serotonergic and adrenergic effects
Mitragynine and the µ-opioid receptor get most of the attention, but kratom’s minor alkaloids reach two other major neurotransmitter systems. They bind serotonin receptors and adrenergic receptors directly — as agonists, partial agonists, or antagonists — and these actions help give the whole plant an effect profile that is not simply opioid-like.
Serotonergic effects. Paynantheine and speciogynine bind the 5-HT₁ₐ and 5-HT₂ᴬ receptors with high affinity. The 5-HT₁ₐ receptor is closely tied to mood and anxiety and to a non-opioid pain-relief pathway: in rodent studies these two alkaloids produce antinociception that an opioid antagonist does not block but a 5-HT₁ₐ antagonist does. This serotonergic layer is likely part of why kratom’s subjective effects can include calm or mood-lift alongside analgesia.
Adrenergic effects. Mitragynine binds α-1 and α-2 receptors at moderate strength, speciogynine binds the α-2 subtypes, and corynantheidine is strongly and selectively α-1D-active. The adrenergic system governs alertness and noradrenergic tone — and α-2 activity in particular is the same mechanism by which clonidine eases opioid-withdrawal symptoms, which connects directly to kratom’s traditional use for managing withdrawal. In every case the alkaloids act directly on the receptors — this receptor-level activity is what shapes kratom’s effects on mood, alertness, and pain.
Mediates opioid-type analgesia, sedation, euphoria, slowed breathing and physical dependence. Agonists activate it; antagonists block it. This is mitragynine’s main target.
5-HT₁ₐ influences mood, anxiety and non-opioid pain pathways; 5-HT₂ᴬ is linked to cardiovascular tissue. Paynantheine and speciogynine bind these with high affinity.
α-1 and α-2 receptors govern alertness, blood pressure and noradrenergic tone. α-2 agonism (like clonidine) eases opioid-withdrawal symptoms — relevant to kratom’s traditional use.
An agonist switches a receptor on; an antagonist binds without activating it and blocks others. The same plant carries both — e.g. corynantheidine blocks the µ-receptor that mitragynine activates.
Speciogynine and speciociliatine share mitragynine’s 2D formula but differ in 3D shape — enough to redirect them toward different receptors.
Corynantheidine blocks the µ-receptor; adding a single –OH at C-9 (giving 9-hydroxycorynantheidine) turns it into a partial agonist.
Consumer caution — alkaloid profiles on kratom labels are frequently unreliable
Kratom is sold as a loosely regulated supplement, and the alkaloid content of leaf, powder and extract products varies enormously between batches, strains and vendors. Certificates of analysis (“COAs”) are often missing, outdated, or inaccurate, and most testing reports only mitragynine and 7-hydroxymitragynine — the minor alkaloids in this chart usually go unmeasured. “Strain” and “color” names on packaging do not reliably predict alkaloid composition. Concentrated extracts and semi-synthetic products can also contain alkaloids at levels far outside anything found in the natural leaf.
Notes
- All molecular formulas and weights are for the neutral (free-base) form, verified with RDKit against published values.
- Speciogynine and speciociliatine are diastereomers of mitragynine: identical 2D connectivity, different 3D configuration. Their drawn structures therefore match mitragynine’s skeleton.
- Abundance figures are typical ranges; real-world content varies widely with plant genetics, harvest and processing.
- This chart covers six of the better-studied minor alkaloids. Kratom contains 40+ alkaloids; many (e.g. corynoxine, isopaynantheine, mitraphylline’s isomers) remain poorly characterised.
- Educational and scientific reference only — not medical, dosing, or safety advice.
Sources (selected)
- León F, Obeng S, et al. Activity of Mitragyna speciosa alkaloids at serotonin receptors. J. Med. Chem. 2021.
- Obeng S, et al. Adrenergic & opioid binding affinities of indole-based kratom alkaloids. J. Med. Chem. 2020.
- Kruegel AC, Grundmann O. Medicinal chemistry & neuropharmacology of kratom. Neuropharmacology 2018.
- Chakraborty S, et al. Metabolism of speciociliatine, an overlooked kratom alkaloid. PMC 2023.
- Reviews: Kratom alkaloids — interactions with enzymes, receptors & cellular barriers. Front. Pharmacol. 2021.
About this chart
Molecular structures rendered with RDKit from isomeric SMILES / InChI; every formula was confirmed by computed molecular formula. Indole-alkaloid numbering follows standard Mitragyna convention.
Receptor-activity ratings are qualitative syntheses of multiple primary studies. Where the literature disagrees (notably speciociliatine at the µ-receptor), the disagreement is shown rather than resolved.
Reference compiled — pharmacology reviewed 2026🩺 Reference, not advice. Activity ratings synthesize multiple in vitro and rodent studies; the literature disagrees on some entries (notably speciociliatine at the µ-receptor), and where it disagrees the chart shows the disagreement rather than picking a winner. Nothing here should be taken as guidance for dosing, identifying products, or self-treatment.