Pharmacology Glossary
Plain-language definitions of scientific terms used throughout the Science Hub.
A
Agonist
A substance that binds to a receptor and activates it. Full agonists produce the maximum possible effect at that receptor. Partial agonists activate it but have a built-in ceiling below the maximum response.
Kratom context: Mitragynine is a partial agonist at mu-opioid receptors, which is part of why its effects differ from morphine.
Antagonist
A substance that binds to a receptor and blocks it without activating it.
Kratom context: Speciociliatine acts as an antagonist at mu-opioid receptors, potentially moderating kratom’s overall opioid effect.
B
Bioavailability
The fraction of a dose that reaches systemic circulation in active form.
Kratom context: Oral bioavailability of mitragynine is estimated at around 3% in animal models, though human data is limited.
C
COA (Certificate of Analysis)
A document from a laboratory reporting the results of testing on a specific product batch. For kratom, a COA should cover alkaloid content, heavy metals, microbiology, and solvent residues.
Kratom context: See the Reading Your COA guide for a full walkthrough.
CYP3A4
A liver enzyme responsible for metabolising mitragynine and many common medications.
Kratom context: Drugs that inhibit CYP3A4 (like fluconazole, ketoconazole, and grapefruit juice) can significantly raise mitragynine blood levels. Drugs that induce it (like rifampin) can reduce them.
E
EC50
The concentration of a substance that produces 50% of its maximum effect at a receptor or in a biological system. Lower EC50 means more potent — it takes less of the substance to produce half the maximum effect.
Emax
The maximum effect a substance can produce at a given receptor, regardless of dose. A partial agonist has a lower Emax than a full agonist.
Kratom context: Mitragynine’s Emax at mu-opioid receptors is approximately 30–44% of the maximum response, compared to near 100% for morphine.
G
G-protein bias / Biased agonism
When an opioid activates the G-protein signalling pathway preferentially over the beta-arrestin-2 pathway. G-protein signalling is associated with analgesia and mood effects; beta-arrestin-2 recruitment is associated with respiratory depression and constipation.
Kratom context: Mitragynine and 7-hydroxymitragynine both show G-protein bias, which may partly explain their different side-effect profile relative to classical opioids — though the clinical significance of this in humans is still being studied.
H
Half-life
The time for the concentration of a substance in blood to fall by 50%.
Kratom context: Mitragynine’s half-life is approximately 9 hours based on published human pharmacokinetic data, meaning effects can last many hours and accumulation is possible with frequent dosing.
I
ISO 17025
The international standard for laboratory accreditation. A COA from an ISO 17025-accredited lab means the testing methods have been independently verified and the lab meets quality management standards.
Kratom context: Always look for this on a kratom COA.
K
Ki (nM)
Binding affinity expressed in nanomolar units. Measures how tightly a substance binds to a receptor: lower Ki means tighter binding and higher affinity. Values above 1000 nM typically indicate weak or negligible binding.
M
MOR / DOR / KOR
The three main opioid receptor subtypes: mu-, delta-, and kappa-opioid receptors. MOR activation drives pain relief, euphoria, and — at high activation levels — respiratory depression. DOR modulates mood and pain. KOR activation is associated with sedation, dysphoria, and analgesia.
Kratom context: Kratom alkaloids interact with all three.
P
Partial agonist
See Agonist. An important additional note: partial agonists can displace full agonists from receptors.
Kratom context: This means kratom can precipitate withdrawal in someone physically dependent on full opioid agonists like oxycodone or heroin.
Pharmacokinetics
How the body absorbs, distributes, metabolises, and eliminates a substance (ADME). Determines onset time, duration of effects, and how much of the substance reaches the bloodstream.
Kratom context: Mitragynine is primarily metabolised by CYP3A4 in the liver.
S
Serotonin syndrome
A potentially life-threatening reaction caused by excess serotonergic activity in the nervous system. Symptoms include agitation, rapid heart rate, high body temperature, and muscle rigidity.
Kratom context: Risk increases when kratom is combined with MAOIs, SSRIs, SNRIs, or tramadol due to mitragynine’s serotonergic activity.