Onset: 15-45 min for liquid/anhydrous; faster (5-15 min) for caffeine gum or buccal lozenges. Tmax 60-90 min. Empty stomach and water-soluble forms are fastest.

Peak: ~1-2 hours. The peak feel is dose-dependent.

Caffeine-naive at 100mg (the user day-1 archetype):

• Sharp jump in alertness ~20-30 min in. Eyes open wider. Subjective "wake-ness" comparable to a great night's sleep on top of an already-rested baseline.
• Mild HR rise (5-15 bpm), faint warm-flush sensation, slight tingle.
• Mood lift, mild euphoria (often more notable than habituated users would predict — naive subjects are essentially "drug-naive" to a CNS stimulant).
• Mild hands-tremor possible at 100-150mg in caffeine-naive; this is what the theanine pairing prevents.
• Cognitive feel: faster context switching, easier to start unpleasant tasks, sustained focus through the 2-4h window.

Caffeine-naive at 100-200mg + L-theanine 200-400mg (the user target protocol):

• Same alertness lift, markedly less peripheral activation — HR rise more like 3-8 bpm, no hand tremor, no jitter, no "wired" sensation.
• EEG signature: tonic alpha-power decrease (focused attention) with sustained calm — the "wakeful relaxation" feel of green tea, scaled up.
• Subjective: "clean clarity" rather than "stim push." This is what most users describe as the ideal cognitive-stack baseline.

Habituated user at 200mg: Mostly reverses withdrawal symptoms; the "lift" is more like getting back to baseline than rising above it. The delta between dose-day and rest-day is small. Tolerance is why a user in this archetype should not slide into daily use.

Plateau: 3-6 hours of clear cognitive runway at 100-200mg. At 400mg+, the plateau extends but the side-effect surface (anxiety, GI, palpitations) grows superlinearly.

Taper: 4-8 hours at typical doses for fast metabolizers; 8-12+ hours for slow metabolizers (CYP1A2 CC). The taper is rarely smooth — most users feel a clean fade unless caffeine-deprived sleep debt has built up, in which case the "crash" is actually unmasked sleep debt.

Honest variability: ~5-10% of users get more anxiety than benefit and don't tolerate caffeine well even at 50mg. ~10-15% are slow metabolizers who feel jittery for hours and sleep poorly even with morning-only dosing. Pharmacogenomic typing (CYP1A2 + ADORA2A) explains a significant chunk of this variance.

• Tolerance buildup: FAST. Adenosine receptor upregulation begins within ~3-7 days of daily dosing; substantial tolerance to alerting + ergogenic effects within 1-2 weeks. This is the single biggest reason caffeine is mis-used in the cognitive-enhancement community — daily users converted a high-leverage PRN tool into a withdrawal-reversal habit.
• Recommended cycle for users in this archetype: 2-4 days on, 3-5 days off pattern (e.g., Mon/Wed/Fri use; Tue/Thu/Sat/Sun off). This preserves the caffeine-naive responder window indefinitely. Daily use erases the user's biggest stack lever within 2 weeks.
• Reset protocol if tolerance develops: 7-14 days complete abstinence is sufficient for adenosine A1/A2A density to renormalize. Day 1-3 are the symptomatic peak (headache, fatigue, irritability — the "withdrawal" experience). Day 4-7 baseline returns. By day 10-14 the pre-tolerance responder state is recovered. Mid-cycle, don't expect partial efficacy returns until day 7+.
• Why the user's cycle protocol is especially important: Two reasons.
  1. Caffeine-naive baseline is a one-time bonus. Once burned, it takes 2+ weeks of full abstinence to recover, and even then the "first dose magic" subjective intensity rarely fully returns.
  2. Late-chronotype migration in progress. Daily caffeine + late-chronotype = sleep architecture damage that compounds over weeks. PRN cycling lets caffeine support the cognitive workload without sabotaging the bedtime-migration project.

Synergistic with

• l-theanine (1:2 ratio, 200mg theanine per 100mg caffeine): The single best-evidenced cognitive-stack pairing in the supplement world. Theanine alpha-wave promotes a "calm-focus" state that smooths caffeine's adrenergic edge while preserving the alerting effect. Multiple A-tier RCTs across attention, mood, anxiety, EEG. Already in the canonical stack. Mandatory pairing for users in this archetype.
• l-tyrosine (500mg-2g, 30-60 min before caffeine): Mechanistically synergistic. Caffeine via A2A blockade increases striatal DA tone; tyrosine supplies the precursor for sustained DA synthesis. Useful for high-cognitive-stress days, sleep-deprived days, sales-call marathons. PRN, not daily.
• Creatine (the canonical stack baseline 5-10g): Neutral-to-synergistic. Old "creatine + caffeine cancel each other ergogenically" claim has been largely debunked in recent meta-analyses; co-administration is fine.
• Beta-alanine (V4 3g): Neutral. Different mechanism (carnosine buffering).
• Citicoline / Alpha-GPC: Cholinergic + caffeine often described as a clean pairing. Already in V4 (citicoline 500mg).

Avoid stacking with

• modafinil (100-200mg) at the same time during onboarding: Additive HR/BP load; both sympathomimetic. Once the user establishes modafinil baseline (post-bloodwork, week 4-8 of modafinil), low-dose caffeine 100mg + 200mg theanine layered on modafinil days is reasonable and historically very common — but not on day 1. Cardiovascular monitoring required during the first 1-2 weeks of combined use.
• High-dose other stimulants (amphetamine, methylphenidate, high-dose synephrine, yohimbine): Cumulative sympathetic load. Anxiety + BP + HR + arrhythmia risk superlinear.
• PM dosing (after 1-4 PM depending on chronotype + CYP1A2 phenotype): Even when subjectively "fine," sleep architecture is degraded. AVOID PM dosing.
• MAOIs (non-selective): Theoretical hypertensive interaction. Selegiline at low MAO-B-selective dose (1-2.5mg) is not a concern.
• Hard training days (archetype-specific): Diaz-Lara MMA literature shows caffeine impairs reaction-time consistency under high arousal in combat sports — even when mean reaction time improves, variance widens. For training where bad reaction-time outliers = punches taken, this matters. Skip caffeine before hard training.

Neutral / safe co-administration

• All the canonical stack supplements (Mg, NAC, citicoline, PS, DHA, curcumin, rhodiola, glycine/tryptophan, D3/K2, beta-alanine, vitamin C) — no interactions of concern.
• Most peptides (BPC-157, TB-500, Semax, Selank, Adamax) — neutral.
• Most Russian nootropics (bromantane, phenylpiracetam, sulbutiamine) — neutral, though phenylpiracetam + caffeine can be over-stimulating in some users.

Caffeine's metabolic profile:

• Primarily metabolized by hepatic CYP1A2 (~95% of caffeine clearance). CYP2E1 contributes minor pathway.
• CYP1A2 is induced by tobacco smoke, cruciferous vegetables, charred meats (PAH activation of AHR receptor → AHR-mediated CYP1A2 upregulation).
• CYP1A2 is inhibited by fluvoxamine (large effect), ciprofloxacin, oral contraceptives, hormonal contraceptives, mexiletine, propafenone.

Clinically significant interactions:

1. Hormonal contraceptives — inhibit CYP1A2, increase caffeine half-life by ~50%. Partner-relevant for users in this archetype, not the user-direct.
2. Fluvoxamine — large CYP1A2 inhibitor; can increase caffeine AUC 5-10×. Avoid combination.
3. Clozapine, olanzapine, theophylline — co-substrates of CYP1A2; caffeine + theophylline = additive bronchodilation + CV load.
4. Modafinil — modafinil weakly induces CYP1A2; chronic modafinil + caffeine can modestly accelerate caffeine clearance.
5. Lithium — caffeine increases lithium clearance via diuresis; relevant for bipolar pharmacotherapy.
6. Adenosine (IV during stress test) — caffeine antagonizes the stress-test response; avoid 24h pre-cardiac stress test.
7. MAOIs (non-selective) — theoretical hypertensive risk; selegiline at MAO-B-selective doses fine.
8. Iron absorption — caffeine reduces non-heme iron absorption when co-ingested with meals; space iron + caffeine 1-2h apart if iron-deficient.

This is where 23andMe (results due ~June 5-15, 2026) becomes load-bearing for users in this archetype's caffeine protocol.

CYP1A2 rs762551 (the dominant pharmacogenomic variant):

• AA genotype = "fast metabolizer" (~40-50% of Caucasians). Caffeine half-life ~3-5h. Cleared from system by 4-6h post-dose. Ergogenic + cognitive benefits maximal; sleep-disruption window shortest. Pre-workout caffeine becomes more reliable; PM cutoff can be earlier (e.g., 6-8h pre-bed).
• AC genotype = "intermediate metabolizer" (~40-50%). Caffeine half-life ~5-8h. Mixed performance benefits.
• CC genotype = "slow metabolizer" (~10-15%). Caffeine half-life 8-10+h. Recent meta-analysis: slow metabolizers actually show worsened performance with caffeine supplementation (likely because the alerting effect persists into rest/recovery + cardiovascular stress accumulates). Slow metabolizers also show higher MI risk on >3 cups/day. For the user-if-CC: caffeine becomes a once-or-twice-weekly tool at most, with 12h pre-bed cutoff.

ADORA2A rs5751876 (adenosine A2A receptor variant):

• TT genotype: more anxiety-prone with caffeine (~80% of caffeine-anxiety variance per some studies).
• CC/CT: less anxiety with same dose.
• 23andMe raw data via Promethease can extract this. If a user in this archetype is TT, theanine pairing is even more important.

AHR rs6968865 / rs4410790 (aryl hydrocarbon receptor):

• T allele = increased CYP1A2 inducibility. T-carriers tend to be heavier habitual coffee consumers (~0.2 cups/day per allele in GWAS).
• Practical impact: T-carriers' CYP1A2 activity is more responsive to inducers (smoke, cruciferous veg, charred meats) → caffeine clearance accelerates with dietary changes more than non-T-carriers.

COMT Val/Val vs Met/Met:

• Val/Val ("warriors") tend to respond more robustly to dopaminergic enhancers including caffeine's A2A→DA disinhibition.
• Met/Met may be more anxiety-prone with caffeine. Already covered in modafinil pharmacogenomics; relevant cross-compound.

Practical archetype-specific recommendation (pre-23andMe):

• Default to "intermediate metabolizer" assumption until June 2026 results.
• Start at 100mg + 200mg theanine, AM only, 2-3 days/week.
• Once 23andMe lands: if AA fast metabolizer, can move to 4 days/week + earlier cutoff fine; if CC slow, drop to 1-2 days/week max with 12h pre-bed cutoff and consider whether caffeine is even worth the sleep cost.

For the user: the V4 Amazon caffeine 200mg tablets are the canonical pick. Pill-cut to 100mg for starter dose. Pair with V4 Suntheanine 200mg theanine cap (already in V4 daily core).

Baseline (before starting)

• Resting HR + BP (3-day morning average) — caffeine adds 5-15 bpm, 3-8 mmHg systolic.
• Subjective sleep quality VAS (Karolinska or simple 1-10) for 7 days pre-dose. Establish caffeine-naive baseline.
• Anxiety baseline (GAD-7 or daily 1-10 VAS).
• Oura/sleep-tracker baseline for 14 nights (REM%, deep sleep%, sleep onset latency, total sleep time).

During use

• First 2 weeks: daily HR/BP morning + post-dose to characterize personal response.
• Daily Oura/sleep tracking — compare on-days vs off-days. Look specifically for deep/REM reduction even if sleep onset latency unchanged — this is where the subjective-objective disconnect hides.
• Weekly subjective cognitive performance VAS on use-days vs rest-days. If the on-vs-off delta shrinks toward zero across weeks, you're in tolerance — increase rest days.
• Anxiety daily VAS — flag any creep upward even at 100mg.
• Once CYP1A2 result lands: recalibrate cutoff time + dose frequency accordingly.

Post-cycle (if cycled / abstinent week)

• Note withdrawal severity (headache, fatigue, irritability) days 1-3. Mild = healthy cycle protocol; severe = was bordering on dependence.
• Sleep-tracker recovery — REM/deep should return within 1-2 nights.
• Cognitive performance baseline check on day 7+ — establishes the "true" caffeine-naive cognitive state for delta comparison.