Caffeine
Adenosine receptor antagonist. The world's most-consumed psychoactive compound. Blocks A1 + A2A adenosine receptors, preventing sleep-pressure accumulation and releasing dopamine/norepinephrine. Cognitive enhancement is the most robustly documented effect in all of pharmacology.
1,3,7-trimethylxanthine. Half-life varies 3–7 hours by CYP1A2 genotype — the single most clinically meaningful pharmacogenomic effect in this catalog. Fast metabolizers (CYP1A2 *1F homozygous) clear caffeine in ~3 hr and may benefit from more frequent, smaller doses; slow metabolizers (CYP1A2 *1A) retain caffeine 5–7+ hr and are at higher cardiovascular risk with regular high intake. The L-theanine 2:1 stack (200 mg theanine + 100 mg caffeine) is the single most consistent nootropic combination in the literature — theanine attenuates jitter without reducing alertness. Tolerance builds within 1–2 weeks of daily use; washout takes ~1 week.
PURE POWDERED CAFFEINE WARNING: FDA has taken action against highly concentrated powders — 1 teaspoon (~3.2 g) is roughly 28 cups of coffee and has caused deaths. If using powder, pre-weigh doses with a milligram scale, never eyeball. CYP1A2 slow metabolizers face higher cardiovascular risk with regular 400+ mg/day intake (elevated blood pressure, arrhythmia risk in susceptible individuals). Tolerance builds fast; cycling preserves effect. Withdrawal headache on cessation is real and lasts 1–3 days.
External links to PubMed searches, ClinicalTrials.gov, and FDA materials. We do not host papers — we point at canonical sources.
GRAS in US. FDA action level for pure powdered caffeine: doses equivalent to 10+ cups of coffee (risk of accidental lethal dosing). Supplement form at standard doses is freely legal everywhere.
Distilled themes from named communities — Reddit threads, forums, creator commentary. Not direct quotes; not clinical evidence. Useful for calibrating expectations against what real self-experimenters report.
Caffeine is the entry-point compound for every nootropic discussion. Cohort consensus: L-theanine 2:1 stack is non-negotiable for daily users — jitter disappears, alertness stays. CYP1A2 genotyping is one of the most practically useful DTC genetic tests because dosing timing directly follows from it. Tolerance cycling (5 days on / 2 days off) is the community gold standard.