A reference work, not a recommendation to use: this profile describes mechanism and evidence and does not replace medical advice. The claims come from marketing and forums and are tested here, not endorsed.

Mechanism of action

Evidence map: Caffeine

Each line stands for a published finding; line style and color show where it comes from. The filter hides weaker evidence levels. Sources: see references.

Caffeine is a nonselective antagonist at adenosine receptors; at usual amounts, its stimulating effect rests on this blockade [1]. A PET study of 15 adults showed that, depending on its concentration in the blood, caffeine occupies 5 to 44% of cerebral A1 receptors; half-maximal displacement was at 67 µM, and with repeated consumption over the day, up to about half of A1 receptors could be occupied [1]. For the wake-promoting effect, the A2A receptor appears to be decisive: mice lacking the A1 receptor became more awake with caffeine, mice lacking the A2A receptor did not [2].

Caffeine is absorbed rapidly and completely and is broken down almost exclusively in the liver via cytochrome P450 enzymes, mainly CYP1A2, to paraxanthine as well as to theobromine and theophylline [3]. The half-life is about 5 hours [1] but varies widely: smoking, oral contraceptives, pregnancy, liver disease, and individual medications change its elimination [3].

It is disputed how much of the effect in habitual consumption is a real benefit. A review of experimental studies concluded that the improvements in performance and mood in regular consumers are almost entirely due to the reversal of withdrawal effects after brief abstinence, and that tolerance develops to the acute effect [4]. Other reviews emphasize the well-documented acute effects on alertness, sustained attention, and reaction time, while memory and higher executive functions respond inconsistently [5].

Targets

TargetTypeActionKey dataEvidence
Adenosine A1 receptorReceptorAntagonisthalf-maximal occupancy at 67 µM in plasma (human, PET) Human, non-randomised[1]
Adenosine A2A receptorReceptorAntagonistmediates the wake-promoting effect (knockout mice) Animal study[2]

Effect profile

Which signaling systems the compound affects according to current data. ↑ enhanced, ↓ dampened, ≈ modulated, ○ no effect; dots = evidence level.

  • Dopamineno data
  • Noradrenalineno data
  • Serotoninno data
  • Acetylcholineno data
  • Glutamate · AMPAno data
  • Glutamate · NMDAno data
  • GABAno data
  • Histamineno data
  • Orexinno data
  • Adenosine↓ · Blocks A1 receptors in the human brain (PET) and, via A2A, the sleep-promoting action of adenosine (mouse)[1][2]
  • Opioidsno data
  • Melatoninno data
  • Neurotrophinsno data
  • Blood flowno data
  • Nitric oxideno data
  • Energy metabolismno data
  • Insulin · glucoseno data
  • Inflammation · oxidativeno data
  • Stress axisno data
  • Testosterone axisno data
  • Endocannabinoidsno data
  • Autophagy · senescenceno data
  • Immune systemno data
  • Lipid metabolismno data
  • Coagulation · fibrinolysisno data
  • Methylationno data

Human studies

  • PET of A1 receptor occupancy

    open-label, one participant with placebo infusion · healthy · n = 15 · Single infusion

    Measured: A1 receptor occupancy

    Result: 5–44% occupancy, concentration-dependent; half-maximal at 67 µM.[1]

    Human, non-randomised

  • Review on withdrawal reversal

    focused review of experimental studies · healthy, habitual consumers and nonconsumers · acute to long-term

    Measured: Performance and mood

    Result: With long-term consumption, hardly any net benefit compared with abstinence; acute effects are subject to tolerance.[4]

    Human, randomised

  • Review of cognitive and physical performance

    narrative review · healthy, including under sleep deprivation · acute

    Measured: Alertness, attention, reaction time, memory, executive functions

    Result: Alertness, sustained attention, attention, and reaction time improved; memory and decisions inconsistent.[5]

    Human, randomised

  • Withdrawal syndrome

    critical review (57 experimental and 9 survey studies) · healthy, habitual consumers · Abstinence up to 9 days

    Measured: Withdrawal symptoms

    Result: Ten validated symptoms, including headache (50%), fatigue, concentration problems; clinically significant impairment in 13%.[6]

    Human, randomised

  • Caffeine and subsequent sleep

    systematic review and meta-analysis (24 studies) · healthy · Single doses before sleep

    Measured: Sleep duration, sleep efficiency, deep sleep

    Result: Total sleep time −45 minutes, sleep efficiency −7%, deep sleep −11.4 minutes.[7]

    Human, non-randomised

  • Pregnancy and birth weight

    meta-analysis of prospective cohort studies (13 studies) · Pregnant women · n = 90747 · Pregnancy

    Measured: Low birth weight

    Result: For each additional cup of coffee per day or so, a 13% higher risk (RR 1.13).[10]

    Human, non-randomised

Risks and warnings

  • Poorer sleep

    Shortens total sleep time by an average of 45 minutes and reduces deep sleep; the effect extends many hours beyond consumption [7].

    Human, non-randomised

  • Withdrawal symptoms

    After stopping in habitual consumers, headache, fatigue, concentration problems, and low mood, usually starting 12–24 hours after the last intake and lasting 2–9 days [6].

    Human, randomised

  • Pregnancy

    Higher consumption was associated with lower birth weight in cohort studies; causality has not been proven [10]. EFSA derived a lower intake without safety concerns for pregnant women than for other adults [9].

    Human, non-randomised

Interactions

  • Fluvoxamine, pipemidic acid, norfloxacin: Markedly inhibit the elimination of caffeine; most other medications hardly affect it [3].
  • Smoking and oral contraceptives: Considerably change caffeine elimination via CYP1A2 [3].

Fact check: common claims

  • supported

    “Caffeine makes you more alert and attentive”

    Acute effects on alertness, attention, and reaction time are well documented in controlled studies [5]; EFSA considered corresponding claims scientifically supported [8].

  • not supported

    “Regular coffee consumption permanently increases mental performance”

    Compared with abstinence, long-term consumption shows hardly any net benefit; much of it is the reversal of withdrawal effects, and tolerance develops to the acute effect [4].

  • partly supported

    “Caffeine improves memory”

    The results on memory and higher executive functions are inconsistent, unlike those on alertness and attention [5].

  • refuted

    “Coffee in the afternoon does not disturb nighttime sleep”

    According to a meta-analysis, caffeine shortens sleep even when it is consumed many hours before bedtime [7].

Pharmacokinetics

Absorption
Rapidly and completely from the gut, without a notable first-pass effect [3].
Half-life
About 5 hours [1], strongly dependent on smoking, contraceptives, pregnancy, and liver health [3].
Metabolism
Almost exclusively liver, mainly CYP1A2; main metabolite paraxanthine, also theobromine and theophylline [3].
Entry into the brain
Occupies adenosine receptors in the human brain (PET) [1].

Legal status

Germany
Food constituent (coffee, tea, energy drinks); status as a medicinal product in tablet form not verified.
EU
In 2015, EFSA assessed the safety of caffeine and derived intakes without safety concerns for adults, pregnant women, and breastfeeding women [9]. EFSA considered health claims on alertness scientifically substantiated; the European Parliament rejected their authorization on health-protection grounds [8].
USA
Not verified.
Other countries
Sport: banned in competition from 1984 to 2004, since then removed from the WADA Prohibited List [9].

Related compounds

Theobrominanalog (German only)Theacrinanalog (German only)

Open questions

  • How large is the real net benefit of caffeine in people who consume it daily?
  • What share do A1 and A2A receptors have in the cognitive effects in humans?
  • Is the association with lower birth weight causal?

References

  1. Elmenhorst D, Meyer PT, Matusch A, Winz OH, Bauer A. Caffeine occupancy of human cerebral A1 adenosine receptors: in vivo quantification with 18F-CPFPX and PET. J Nucl Med. 2012;53(11):1723–9. doi:10.2967/jnumed.112.105114 · PubMed
  2. Huang ZL, Qu WM, Eguchi N, Chen JF, Schwarzschild MA, Fredholm BB, et al. Adenosine A2A, but not A1, receptors mediate the arousal effect of caffeine. Nat Neurosci. 2005;8(7):858–9. doi:10.1038/nn1491 · PubMed
  3. Grzegorzewski J, Bartsch F, Köller A, König M. Pharmacokinetics of caffeine: a systematic analysis of reported data for application in metabolic phenotyping and liver function testing. Front Pharmacol. 2022;12:752826. doi:10.3389/fphar.2021.752826 · PubMed
  4. James JE, Rogers PJ. Effects of caffeine on performance and mood: withdrawal reversal is the most plausible explanation. Psychopharmacology (Berl). 2005;182(1):1–8. doi:10.1007/s00213-005-0084-6 · PubMed
  5. McLellan TM, Caldwell JA, Lieberman HR. A review of caffeine's effects on cognitive, physical and occupational performance. Neurosci Biobehav Rev. 2016;71:294–312. doi:10.1016/j.neubiorev.2016.09.001 · PubMed
  6. Juliano LM, Griffiths RR. A critical review of caffeine withdrawal: empirical validation of symptoms and signs, incidence, severity, and associated features. Psychopharmacology (Berl). 2004;176(1):1–29. doi:10.1007/s00213-004-2000-x · PubMed
  7. Gardiner C, Weakley J, Burke LM, Roach GD, Sargent C, Maniar N, et al. The effect of caffeine on subsequent sleep: a systematic review and meta-analysis. Sleep Med Rev. 2023;69:101764. doi:10.1016/j.smrv.2023.101764 · PubMed
  8. de Boer A. Fifteen years of regulating nutrition and health claims in Europe: the past, the present and the future. Nutrients. 2021;13(5):1725. doi:10.3390/nu13051725 · PubMed
  9. Diel P. Caffeine and doping – what have we learned since 2004. Nutrients. 2020;12(8):2167. doi:10.3390/nu12082167 · PubMed
  10. Chen LW, Wu Y, Neelakantan N, Chong MF, Pan A, van Dam RM. Maternal caffeine intake during pregnancy is associated with risk of low birth weight: a systematic review and dose-response meta-analysis. BMC Med. 2014;12:174. doi:10.1186/s12916-014-0174-6 · PubMed
How this profile was checked

Last reviewed: 2026-10-23. All sources with a DOI or PMID were resolved automatically via Crossref or doi.org and PubMed.

Translated from the German profile (version of 2026-10-23); translation checked on 2026-10-26.

Checked

  • All 10 DOIs resolved via Crossref and all PMIDs via PubMed with tools/pruefe_profile.py.
  • Abstracts: nonselective antagonist, n = 15, 5–44%, 67 µM, up to 50% with repeated consumption, t½ ≈ 5 h [1]; A2A vs. A1 knockout [2]; CYP breakdown to paraxanthine, theobromine, theophylline [3]; withdrawal reversal, tolerance [4]; alertness, attention, reaction time, inconsistent for memory [5]; 57 + 9 studies, headache 50%, 13%, onset 12–24 h, duration 2–9 d [6]; 24 studies, −45 min, −7%, −11.4 min deep sleep [7]; RR 1.13 per increment, 90,747 participants [10].
  • In the full text: fluvoxamine, pipemidic acid, norfloxacin inhibit caffeine clearance; influence of smoking and contraceptives [3]; EFSA considered four alertness claims substantiated, the Parliament rejected them [8]; EFSA safety opinion 2015 and WADA removal 2004 [9].
  • ClinicalTrials.gov: 207 entries for “caffeine cognition” (October 23, 2026).

Not verified

  • The EFSA opinions themselves (DOI 10.2903/j.efsa.2015.4102 and 10.2903/j.efsa.2011.2054) were not retrievable as full text; their content is documented only via [8] and [9].
  • The specific EFSA amounts without safety concerns were deliberately not adopted because of the “no dosages” rule.
  • The direction of the influence of smoking and contraceptives on clearance was not found verbatim in the text checked; hence only “change”.
  • Designs of the 24 sleep studies [7] not checked individually (hence evidence level 3).

No dosages, no sources of supply. This information does not replace medical advice.

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