Psychedelic and Psychopharmacology Glossary

The Psychedelic and Psychopharmacology Glossary defines the terms used across the Library's plant-medicine and harm-reduction material and in works like PIHKAL and TIHKAL. Definitions are written in the Library's own words for reference and education; for the full compound entries the Library links out to Erowid (see PIHKAL and TIHKAL). It is part of the Glossaries section.

This is reference, not advice or encouragement. Where a term touches safety, the safer-practice meaning is given.

A

Agonist — a molecule that binds a receptor and activates it, producing an effect. Most classic psychedelics are agonists (or partial agonists) at a particular serotonin receptor.
Antagonist — a molecule that binds a receptor but blocks it, preventing activation.
Afterglow — a mood/clarity aftereffect some people report in the hours or days after an experience, once the acute effects have passed.

B

Bioassay — testing a substance's effect by taking a measured dose and recording the result. Alexander Shulgin's careful, low-to-high self-bioassays are the method behind PIHKAL and TIHKAL.
Bad trip / challenging experience — a frightening or distressing episode during a psychedelic experience; harm-reduction practice reframes it as something to move through with calm support, not fight.

C

Come-up / onset — the period as effects begin and build.
Contraindication — a condition or combination that makes a substance unsafe for a given person (e.g. certain medications, heart conditions, pregnancy). Checking contraindications is a core safety step.
Cross-tolerance — when tolerance to one substance also reduces the effect of a related one (common among classic psychedelics), so taking them close together does not "stack" the way people expect.

D

Deliriant — a class (e.g. certain anticholinergics) that produces confusion and true, often unpleasant, hallucinations; distinct from — and generally far riskier than — classic psychedelics.
Dissociative — a class (e.g. ketamine, DXM) that produces a sense of detachment from body/environment; a different mechanism from classic psychedelics.
Dose-response — the relationship between how much is taken and the effect; small differences in dose can mean large differences in experience, which is why start-low is the harm-reduction rule.
Duration — total length of effects, from onset through comedown; recorded alongside dose for every compound.

E

Entactogen / empathogen — a class (e.g. MDMA) whose signature is emotional openness, warmth, and connection rather than strong visual change. Many are phenethylamines.

M

MAOI (monoamine oxidase inhibitor) — a substance that blocks an enzyme which breaks down certain molecules. MAOIs can dangerously potentiate other drugs and foods and are a major source of interactions (the basis of ayahuasca's activity, and a serious contraindication with many medicines).
Microdose — a sub-perceptual dose (well below a felt threshold) taken on a schedule; evidence for benefits is mixed and still debated.

P

Phenethylamine — the chemical family at the center of PIHKAL (the 2C-x and DOx series, the mescaline and MDMA relatives).
Potentiation — one substance amplifying another's effect; can be intended or a hidden danger (see MAOI).
Prodrug — a compound inactive until the body converts it into the active form (e.g. psilocybin → psilocin).
Psychedelic (classic / serotonergic) — substances (e.g. LSD, psilocin, mescaline, DMT) that act mainly as agonists at the 5-HT2A serotonin receptor, producing changes in perception, thought, and mood.

R

Reagent testing — using chemical test kits (Marquis, Mecke, Simon's, etc.) to check what a substance actually is before use; a cornerstone of harm reduction because street material is frequently mis-sold.
Racemate / enantiomer — mirror-image forms of the same molecule; they can differ sharply in activity, a recurring theme in Shulgin's chemistry.

S

Set and setting — one's mindset (mood, intention, expectations) and the physical/social environment. Widely held to shape an experience as much as the substance and dose.
Shulgin Rating Scale — Shulgin's qualitative intensity scale, from minus (no effect) through +, ++, +++ to the rare ++++ (a distinct peak/transcendent state); see PIHKAL and TIHKAL.
Structure–activity relationship (SAR) — how small changes to a molecule's structure change its effect; the systematic thread running through PIHKAL and TIHKAL.
Serotonergic — acting on the serotonin system; the mechanism of the classic psychedelics.

T

Test dose / allergy dose — taking a small fraction first to check for a bad reaction before a full dose.
Threshold dose — the smallest amount at which any effect is felt (the "±" on the scale).
Tolerance — reduced effect from repeated use, so more is needed for the same result; for classic psychedelics it builds fast and fades after a break.
Tripsitter — a sober, trusted person who stays present to keep someone safe and grounded during an experience.
Tryptamine — the chemical family at the center of TIHKAL (DMT and relatives, the psilocin group).

Sources

Terminology reflects standard pharmacology and harm-reduction practice; compound-specific data and the rating scale trace to A. & A. Shulgin's PIHKAL and TIHKAL, with full entries hosted at Erowid.