Compound or practice · Supplement
Caffeine
The most-used and best-evidenced cognitive and physical stimulant on earth — precise dosing and a hard afternoon cutoff are the whole skill.
- Overall grade
- Grade A
- Last revised
- 4
- graded outcomes
- 0
- references listed
A structured monograph in Prometheus's bundled library. Its grades are the monograph's own reading of the evidence, not a peer-reviewed review.
3 at grade A · 1 at grade B — 4 graded rows
What is known
Each outcome with its grade (how strong the evidence is, A to D), the size of the effect where the entry records one, and how often studies agree.
- Alertness / reaction timeEvidence grade A
Acute, tolerance-limited
Moderatehigh
- Endurance performanceEvidence grade A
Well established
Moderatehigh
- Sleep (afternoon dosing)Evidence grade A
Degrades sleep — the key cost
Moderatehigh
- Strength / powerEvidence grade B
Smaller effect
Smallmixed
What is uncertain
Read off the same rows as above. Nothing here is added that the entry does not itself record.
- Studies disagree: Strength / power (mixed agreement).
- Caution with Rhodiola Rosea: Both activating — stack low-dose.
- Watch with Melatonin: Opposing circadian signals — the afternoon cutoff protects the melatonin window.
- Watch with Iron: Cuts non-heme iron absorption ~40% — separate by 2 hours.
- Watch with Rhodiola Rosea: Both activating — stack low-dose.
Interactions
Documented pairings with supplements and medicines. A medicine on this list is a conversation for whoever prescribes it.
Rhodiola RoseaCaution
Both activating — stack low-dose.
MelatoninMind the timing
Opposing circadian signals — the afternoon cutoff protects the melatonin window.
IronMind the timing
Cuts non-heme iron absorption ~40% — separate by 2 hours.
L-TheanineWorks together
The A-grade pairing — focus without the jitter.
The evidence in words
What it is
Caffeine is a plant alkaloid and the world's most consumed psychoactive drug. It reliably increases alertness, reduces perceived effort, and improves endurance and some strength and reaction-time measures.
How it works
It blocks adenosine receptors, masking the 'sleep pressure' adenosine builds through the day, which sharpens alertness and lowers perceived exertion. Its ~5-hour half-life is why late doses wreck sleep.
What the evidence shows
Grade A for acute alertness and for endurance and repeated-effort performance; strength benefits are smaller. Tolerance blunts the alertness boost with daily use, and the sleep cost of afternoon dosing is the single biggest lever most people ignore. Pairing with L-theanine reduces jitter.
How to use it
1–3 mg/kg (roughly 80–200 mg) for focus or ~3–6 mg/kg pre-exercise. Hard cutoff by early afternoon — caffeine within ~8–10 hours of bed measurably degrades sleep even when you fall asleep fine.
Safety & interactions
Safe for most in moderation; excess causes anxiety, palpitations, and insomnia. Dependence and withdrawal (headache, fatigue) are real. Combining multiple stimulants compounds cardiovascular strain. L-Theanine — The A-grade pairing — focus without the jitter. Melatonin — Opposing circadian signals — the afternoon cutoff protects the melatonin window. Iron — Cuts non-heme iron absorption ~40% — separate by 2 hours. Rhodiola Rosea — Both activating — stack low-dose.
Bottom line
A genuinely effective, cheap ergogenic and nootropic. Dose to purpose, respect the early-afternoon cutoff, and pair with theanine if it makes you jittery.
How it is used
As the entry records it, from the studies it summarises. Not a recommendation for you — doses that suit a trial population may not suit you.
- Focus
- 80–200 mg; theanine 200 mg to smooth it.
- Pre-exercise
- 3–6 mg/kg, ~45–60 min prior.
Goes with
- L-TheanineThe A-grade pairing — focus without the jitter.Evidence grade B
Related reading
Entries that share this one's tags or conditions.
- Caffeine for Alertness / reaction timeEvidence grade A — Acute, tolerance-limitedEvidence grade A
- Caffeine for Endurance performanceEvidence grade A — Well establishedEvidence grade A
- Caffeine for Sleep (afternoon dosing)Evidence grade A — Degrades sleep — the key costEvidence grade A
- Caffeine for Strength / powerEvidence grade B — Smaller effectEvidence grade B
Sources
This entry lists no references of its own.
Last revised . A grade is this entry's reading of the evidence, not medical advice.