Caffeine

Caffeine is the most widely used psychoactive drug on the planet. It blocks adenosine, the molecule your brain accumulates over the day to make you feel tired, which is why a cup of coffee makes you feel less sleepy, more alert, and a bit sharper. That's the entire pitch and it's the reason about 80% of adults consume it daily.

Most people use it as a daily stimulant for energy, focus, and mood. It also genuinely improves endurance, strength output, and reaction time, so it doubles as the most evidence-backed pre-workout on the market. The catch is that tolerance builds within days to weeks, sleep is the first casualty if you mistime it, and the dose that works well for a 90kg man can be way too much for a 60kg woman or anyone on hormonal contraceptives that slow caffeine clearance.

Deep-dive

Caffeine is an adenosine receptor antagonist. As you stay awake, adenosine builds up in your brain and binds to A1 and A2A receptors, which dampens neural firing and increases the subjective feeling of sleepiness. Caffeine has a similar enough shape to slot into those same receptors without activating them, so it blocks adenosine from doing its job. The result isn't extra energy, it's the temporary removal of a brake. Downstream of that disinhibition, dopamine signalling in the striatum rises, noradrenaline release climbs, and you get the familiar pattern: more alert, faster reaction time, slightly better mood, mildly elevated heart rate and blood pressure.

Plasma caffeine peaks 30-60 minutes after ingestion and has a half-life of around 5 hours in a typical adult, meaning if you drink a strong coffee at 2pm, roughly half of that caffeine is still in your bloodstream at 7pm and a quarter at midnight. Half-life varies massively between people though, from about 2 hours in fast metabolisers to 10+ hours in slow ones. The main driver is the CYP1A2 enzyme in the liver, which has a well-studied genetic polymorphism. Slow metabolisers (the AC or CC genotype) clear caffeine roughly 40% slower than fast metabolisers (AA), and a 2006 study in the Journal of the American Medical Association found slow metabolisers had a higher risk of nonfatal heart attack with high caffeine intake, while fast metabolisers showed no such association. This is why some people can drink espresso after dinner and sleep fine and others get wired off a single morning cup.

Cognitive performance. A 2010 Cochrane review of caffeine in shift workers concluded it consistently improved attention, reaction time, and reasoning, with effects comparable to a nap. A 2014 meta-analysis on sleep-deprived subjects found doses of 200-600 mg restored cognitive function close to baseline. The effects on already-rested people are smaller but real: reaction time and vigilance reliably improve at 100-400 mg. Effects on more complex executive function are mixed and may even be slightly negative at high doses in people who are already at peak alertness. The pattern is similar to tyrosine in one respect, the bigger your alertness deficit, the bigger the gain.

Physical performance. This is one of the most replicated ergogenic effects in sports science. A 2018 umbrella review covering 21 meta-analyses confirmed moderate benefits for endurance (around 2-4% improvement in time-trial performance), strength (small but consistent 2-7% improvement in 1RM and reps), power output, and sprint performance. The effective dose for performance is 3-6 mg/kg, taken 30-60 minutes before exercise, which works out to roughly 200-400 mg for most people. Higher doses don't help and start producing side effects. Mechanism is a mix of central (reduced perception of effort) and peripheral (increased calcium release in muscle, fatty acid mobilisation). Habitual users still benefit, the old idea that you need to abstain for a week to restore the effect hasn't held up in controlled trials.

Tolerance. Tolerance to the alertness and mood effects develops within 1-4 weeks of daily use, partly through receptor upregulation (your brain makes more adenosine receptors to compensate for the blockade). The ergogenic and physical performance effects are more resistant to tolerance. The cardiovascular effects (heart rate, blood pressure bump) also blunt with chronic use. This means daily caffeine users mostly aren't getting a stimulant, they're using it to feel normal, the morning cup is reversing the mild withdrawal that built up overnight. Whether that matters depends on your goals. If you want caffeine to work as an acute lever (before a hard session, a long drive, a critical meeting), you need to be at low chronic intake. If you just want to enjoy coffee daily and accept that the perceived effect is partly withdrawal reversal, that's fine too.

Sleep. This is where caffeine quietly does the most damage. A 2013 trial gave 12 healthy adults 400 mg caffeine at bedtime, 3 hours before bed, or 6 hours before bed, then measured sleep with polysomnography. All three timings significantly disrupted sleep compared to placebo, including the 6-hour-before dose, which reduced total sleep time by over an hour. People consistently underestimate this because subjectively they fall asleep fine, but objective sleep quality (deep sleep specifically) drops. The 6-hour rule is a reasonable minimum for most people, fast metabolisers may get away with less, slow metabolisers need 8-10 hours of clearance.

Mood and mental health. Light to moderate caffeine intake is associated with lower depression risk in large prospective cohorts, though this is observational and confounded by reverse causation (depressed people tend to drink less coffee). Higher doses can worsen anxiety, especially in people predisposed to panic, and genetic studies suggest the adenosine A2A receptor variant influences who feels anxious off caffeine. If coffee makes you jittery and anxious at doses other people tolerate fine, you probably have the sensitive variant. Lower the dose, don't fight it.

Women. Caffeine clearance is significantly slower during the luteal phase of the menstrual cycle, in the second and third trimesters of pregnancy, and on combined oral contraceptives, which roughly double the half-life by inhibiting CYP1A2. A pharmacokinetics study found women on oral contraceptives cleared caffeine at about half the rate of women not on them, meaning the same morning espresso lingers significantly longer. Practically, women on the pill should treat 200 mg of caffeine like 400 mg in terms of duration and sleep timing. Pregnancy guidelines from most major bodies cap intake at 200 mg/day, the data on miscarriage and low birth weight above that threshold is mixed but consistent enough to warrant caution. Postmenopausal women lose some of the cycle-dependent clearance variation but generally clear caffeine slightly slower than men of the same age. Bone density is the other consideration, very high caffeine intake (>400 mg/day) combined with low calcium intake has been associated with modest bone density loss in postmenopausal women, but the effect disappears when calcium intake is adequate.

Cardiovascular and metabolic. Acutely, caffeine raises blood pressure by 5-10 mmHg systolic for a few hours. This blunts with regular use. Chronically, the largest cohort studies show 3-5 cups of coffee per day is associated with lower all-cause mortality, lower cardiovascular disease, and lower type 2 diabetes risk. This is observational and the protective compounds in coffee (polyphenols, chlorogenic acids) are doing some of the work, but isolated caffeine in moderate doses doesn't appear to harm a healthy cardiovascular system. People with poorly controlled hypertension or arrhythmias are the main exception.

Withdrawal. Real, predictable, and unpleasant. Headache, fatigue, low mood, and difficulty concentrating peak around 24-48 hours after the last dose and resolve within 7-9 days. Tapering by 25-50 mg per day rather than going cold turkey makes it almost symptom-free.

Dosage:

Standard alertness dose: 50-200 mg, depending on body weight and tolerance. A regular cup of brewed coffee is around 95-200 mg, an espresso shot 60-80 mg, a strong energy drink 150-300 mg

Cutoff time: Stop at least 6 hours before your intended sleep time, 8-10 hours if you're a slow metaboliser, on hormonal contraceptives, or pregnant. If you're not sure which camp you're in, default to the longer window

Daily ceiling for healthy adults: 400 mg/day is the commonly cited upper limit. Pregnancy: 200 mg/day. Slow metabolisers and people with anxiety, arrhythmia, or hypertension should sit well below these numbers

Tolerance management: If you want caffeine to work as an acute lever, cap daily intake at 100 mg or less and save the higher doses for when you need them. If you want to use it daily for general energy, accept the trade-off and don't expect it to function as a stimulant anymore

Tapering off: Drop by 25-50 mg per day over 1-2 weeks. Cold turkey works but produces 5-7 days of headache, fatigue, and irritability

Forms: Coffee is the most pleasant delivery and brings polyphenols along for the ride. Tea is gentler and has theanine built in. Caffeine pills (100 or 200 mg) are useful when you want precise dosing or no liquid. Energy drinks are fine but watch the sugar and additives. Pre-workout powders vary wildly in dose, read the label

Here's what you can expect:

Within 30-60 minutes of a 100-200 mg dose, expect to feel more alert, slightly more focused, and in a marginally better mood. Reaction time improves, perceived effort during exercise drops, and tasks that felt boring feel a bit less so. The peak lasts 1-3 hours and trails off over the next several hours. If you take it before a workout, you'll usually feel like you can push slightly harder for slightly longer.

If you're a daily user, the subjective stimulant effect is mostly gone. The morning cup mostly reverses the mild withdrawal that built up overnight, returning you to your normal baseline rather than lifting you above it. The performance effects (endurance, strength, reaction time) are more resistant to tolerance and stay useful.

If you take too much, or you're a slow metaboliser, or you've stacked it with something else stimulating, expect jitters, racing heart, anxiety, GI distress, and a crash 4-6 hours later. The crash is mostly sleep debt catching up plus blood sugar dipping, not caffeine withdrawal in the technical sense.

Side effects & risks:

Jitters, anxiety, and racing heart at doses above your personal tolerance. Common with 400+ mg in non-habitual users or anyone with the sensitive A2A genotype. Drop the dose

Sleep disruption is the most underrated side effect. People routinely sleep worse without realising it because they fall asleep fine. Deep sleep takes the biggest hit. Strict cutoff times matter more than people think

GI distress: stomach upset, loose stools, and reflux are common, especially on an empty stomach. Coffee in particular is a known reflux trigger

Headache and irritability during withdrawal, peaking 24-48 hours after the last dose. Resolves within a week. Taper to avoid it

Blood pressure rises 5-10 mmHg systolically for a few hours after a dose. Blunts with regular use, but uncontrolled hypertension is a reason to keep intake low

Arrhythmias: moderate caffeine is fine in most people with healthy hearts, but anyone with diagnosed atrial fibrillation, SVT, or other arrhythmia should clear intake with their cardiologist

Anxiety and panic disorders: caffeine can trigger panic attacks in susceptible people. If you have panic disorder, very low intake or abstinence is the conservative move

Pregnancy: keep intake under 200 mg/day. Higher intake has been associated with miscarriage and low birth weight in observational data

Hormonal contraceptives roughly double caffeine half-life. Same dose, twice the duration. Adjust accordingly

Medication interactions: caffeine interacts with a long list of drugs via CYP1A2, including some SSRIs (fluvoxamine in particular, which dramatically slows caffeine clearance), quinolone antibiotics, theophylline, and lithium. If you're on regular medication, check

Overdose: genuinely possible. Roughly 10 g of caffeine (the equivalent of 50-100 cups of coffee, or a poorly measured powdered caffeine dose) can be lethal. This is essentially a powdered caffeine problem. Don't dose powder by eye

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Blood markers

Blood pressure, baseline. Caffeine acutely raises BP, and if you're using it daily plus combining with other stimulants or pre-workout supplements, you want a reference point. Recheck every 6-12 months if intake is high.

Resting heart rate and ideally a 24-hour ECG or wearable rhythm check if you notice palpitations or have any cardiac history. Caffeine isn't a problem in healthy hearts but can unmask or worsen existing arrhythmia.

CYP1A2 genotype is the most useful one-time test if you're optimising for performance or sleep. Knowing whether you're a fast or slow metaboliser tells you how aggressive your cutoff time needs to be and roughly how much you can tolerate. Available through 23andMe raw data, Nutrigenomix, or dedicated CYP1A2 panels.

Ferritin and iron studies if you drink coffee or tea with meals. Caffeine and the polyphenols in coffee/tea inhibit non-heme iron absorption by up to 50% when consumed within an hour of food. Women of menstruating age are most at risk, and a low ferritin with no other cause often resolves by moving coffee away from meals.

TSH and free T4 if you're consuming high amounts daily and feel persistently wired-but-tired. Heavy caffeine intake doesn't directly cause thyroid issues but can mask or mimic them, so baseline thyroid function is worth knowing.

For most people drinking 1-3 cups a day with no symptoms, no specific bloodwork is needed. The people who actually benefit from testing are heavy daily users, anyone with cardiac symptoms, women on hormonal contraception, and anyone who's noticed they feel anxious or jittery on doses that others tolerate fine.

Sold as a beverage and dietary supplement worldwide without prescription. Powdered pure caffeine is sold legally in many countries but is significantly more dangerous than coffee or pills due to dosing errors.