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Caffeine

Performance

StrongFor the population and outcome described, multiple large human trials broadly agree.

The most effective legal stimulant, and the easiest one to build tolerance to.

3-6 mg/kg
The dose range across performance trials, ~200-400 mg for most adults, taken ~60 min before

What it is

The most-used performance drug on earth, and one of the very few supplements where the effect is large, reliable and replicated across hundreds of trials. The interesting questions are not whether it works (it does) but dose, timing, tolerance and sleep cost.

How it works

Caffeine blocks adenosine receptors, adenosine being the molecule that accumulates while you are awake and signals fatigue. Blocked receptors mean less perceived effort and fatigue, faster reaction time, and measurably better endurance and strength output. It is a loan against alertness, not free energy: the adenosine is still there when the caffeine leaves.

Which form to buy

CoffeeDaily life

Dose varies wildly per cup (~60-200 mg). The ritual and polyphenols are bonuses; the imprecision is the tradeoff.

Great for living, imprecise for dosing.

Caffeine tablets (100-200 mg)Training & precision

Exact doses, cheap, what most performance trials actually used.

The performance tool.

Pre-workout blendsReading labels carefully

Often 200-400 mg alongside a proprietary blend of everything else. The caffeine is usually the only ingredient doing heavy lifting.

You are mostly buying expensive caffeine.

Energy drinksConvenience with baggage

80-300 mg plus sugar and price.

Works; costs more per mg than anything else here.

Caffeine + L-theanineThe jitter-prone

Theanine takes the edge off the wired feeling in small studies without killing alertness.

A legitimate combo if plain caffeine makes you anxious.

What it actually does, graded

Endurance performanceStrong

One of the most replicated findings in sports science, ~2-4% improvements at 3-6 mg/kg.

Strength & power outputStrong

Smaller than endurance effects but consistent across meta-analyses.

Alertness & reaction timeStrong

Robust, especially when sleep-deprived.

Fat lossLimited

A real but small metabolic bump that tolerance erodes, nobody gets lean from caffeine.

More total energy over the dayNone shown

It defers fatigue; it does not create energy. The bill arrives, usually at night.

Pros and cons

Pros
  • Large, reliable, immediately noticeable effect, rare in this catalog
  • Extremely cheap in tablet form
  • Decades of safety data at sensible doses
Cons
  • Sleep cost is the hidden tax: a 5-6 hour half-life means afternoon doses are still active at midnight
  • Tolerance builds within weeks, shrinking the benefit you keep paying for
  • Anxiety, jitters and heart pounding are dose-dependent and real
  • Withdrawal headaches are genuine, the "caffeine fixes my headache" loop is often just deferred withdrawal

What to expect

  • Within 30-60 min: the peak, alertness, drive, better training output.
  • Weeks of daily use: tolerance blunts most of the subjective kick; performance effects partially survive.
  • Skipping a day after habitual use: withdrawal headache and fog, typically day 1-2, gone within a week.

Interactions

TimingSleepThe one that matters most. Half-life averages ~5-6 hours but varies a lot by person: a 3pm 200 mg dose still leaves ~100 mg working at 9pm and ~50-65 mg near midnight. A 400 mg trial found impaired sleep even taken six hours before bed, there is no universal safe cutoff, just earlier being better.
CautionAnxiety disordersCaffeine reliably worsens anxiety symptoms; panic-prone people are meaningfully more sensitive.
CautionFluvoxamineA strong interaction: it slowed caffeine clearance ~fivefold in study, stretching the half-life from ~5 to ~31 hours, one coffee can behave like an all-day drip. Prescriber territory.
MonitorCiprofloxacinA moderate version of the same effect (~1.6x exposure, half-life ~5→8 h in one study).
CautionPregnancyGuidance generally caps intake around 200 mg/day, clearance slows dramatically in pregnancy.
MonitorBlood pressureAcute readings rise post-dose; habitual use partially adapts. Worth knowing if you home-monitor.
CompatibleCreatineThe old interference claim rests on thin data; practical consensus is they coexist fine.

Is it for you?

Probably worth it if
  • Training sessions and competition where output matters (trials typically dosed ~60 min before)
  • Shift work and sleep-deprived stretches, used strategically
  • You want one supplement whose effect you can actually feel
Probably skip it if
  • Your sleep is already broken, fix that first; caffeine is likely part of the loop
  • Anxiety runs hot, this reliably makes it hotter

Who should avoid it

  • Uncontrolled hypertension or serious arrhythmias, without clinician input
  • Pregnancy above ~200 mg/day

Research on this supplement

Papers about this supplement, not proof of the claims above. A title says what was studied, which is sometimes a negative result or a different question entirely.

  • The effect of caffeine on subsequent sleep: A systematic review and meta-analysis
    Sleep Med Rev, 2023
  • Effects of Caffeine Intake on Endurance Running Performance and Time to Exhaustion: A Systematic Review and Meta-Analysis
    Nutrients, 2022
  • Caffeine and Cognitive Functions in Sports: A Systematic Review and Meta-Analysis
    Nutrients, 2021

What the evidence says

Very large human trial base for endurance and alertness. Tolerance to the alertness effect develops within days.

What studies used

100-300 mg; 3-6 mg/kg in performance trials. Cut it 8-10 h before bed, the half-life is ~5 h.

Reported from published studies and product labelling — a record of what was used in research, not a recommendation. What is right for you is a conversation with a clinician.

Watch out for

  • Wrecks sleep long after you stop feeling it
  • Stacks silently across coffee, pre-workout and energy drinks

Evidence base

S42,380 research papers
established literature

Large human evidence base: several phase 3 or 4 trial reports, plus multiple meta-analyses or systematic reviews.

This letter measures how much human research exists, not whether it works for you — the tier above (Strong) is the read on quality.

Research (12)

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Where next

Every supplement in the catalogThe full shelf, grouped and evidence-tiered.→The compound indexSteroids, nootropics and research chemicals.→The peptide indexInjectable peptides and what trials used.→