Caffeine works by blocking the brain's sleepiness signal. Its best-supported effects are sharper attention and reaction speed, better endurance and, to a smaller degree, strength. Lower Parkinson's disease and type 2 diabetes rates are consistent but untested in prevention trials. The costs — lighter sleep, higher blood pressure, anxiety in susceptible people, withdrawal — are more certain than several benefits. (Full Review)

Protocol

Standard performance dose

3–6 mg per kilogram

Taken 45–60 minutes before exercise

Standard cognitive dose

100–200 mg

On waking or mid-morning

Best time of day

Morning to early afternoon

A midday dose still leaves roughly a quarter circulating at bedtime

Time to effect

Subjective alertness

15–30 minutes

Peaks at 30–120 minutes

Performance effects

15–30 minutes

Same curve as alertness, peaking at 30–120 minutes

Body composition and metabolic

Weeks to months

Before effects become measurable

Benefits

High: Endurance exercise performance; muscle strength and power; alertness, attention and reaction time; enhanced analgesic effect; improved airway function

Medium: Lower risk of Parkinson's disease; reduced body weight and fat mass; lower risk of type 2 diabetes

Low: Lower risk of depressive symptoms; lower all-cause mortality (conflicted); preserved cognition and dementia risk (conflicted)

Speculative: Direct lifespan extension

Contraindications

Panic disorder or treatment-resistant generalised anxiety disorder

Chronic insomnia meeting diagnostic criteria, or diagnosed sleep-onset disorders

Uncontrolled hypertension (above 160/100 mmHg despite treatment)

Symptomatic arrhythmia, including ventricular ectopy causing symptoms

Pregnancy above 200 mg daily; attempting conception above 300 mg daily

Severe hepatic impairment (Child-Pugh Class C)

Concurrent clozapine or theophylline therapy without drug-level monitoring

Key Interactions

CYP1A2 inhibitors (fluvoxamine, ciprofloxacin, oral contraceptives, cimetidine)

CYP1A2 inducers (smoking, carbamazepine, rifampicin)

Lithium

Stimulant medications (amphetamines, methylphenidate, pseudoephedrine)

Over-the-counter combination analgesics (Excedrin, some cold remedies) and nicotine

Supplement interactions (synephrine, yohimbine, ephedra-type botanicals, high-dose green tea extract)

Additive supplements sharing caffeine's targets (theophylline-containing botanicals, theobromine from cocoa)

Other intervention interactions (time-restricted eating, cold exposure, sauna)

Risk & Side Effects

High: Sleep disruption; physical dependence and withdrawal; elevated blood pressure; anxiety and panic provocation

Medium: Acute impairment of glucose tolerance; increased ventricular ectopic beats; increased risk of pregnancy loss

Low: Reduced bone mineral density and fracture risk (conflicted); raised intraocular pressure in genetically susceptible people; gastro-oesophageal reflux and gastrointestinal irritation; acute caffeine toxicity

Speculative: Blunted brain adaptation to sleep restriction

Monitoring

Marker	Target	Why
Systolic / diastolic blood pressure	Below 115/75 mmHg	Caffeine's most consistent adverse effect on a hard clinical measure
Resting heart rate	50–65 beats per minute	Detects excess stimulant load from stacked products
Fasting glucose	75–85 mg/dL	Caffeine acutely impairs post-meal glucose disposal
HbA1c	4.8–5.4%	Confirms whether acute glucose effects accumulate
Fasting insulin	Below 5 µIU/mL	More sensitive than glucose to the insulin-resistance signal seen acutely with caffeine
24-hour urinary calcium	100–250 mg/day	Tests the proposed route from caffeine to bone loss
Intraocular pressure	10–21 mmHg	Relevant only where family history or genetic risk of glaucoma exists
CYP1A2 genotype (rs762551)	No established target; the result is categorical	Distinguishes fast from slow caffeine clearance
ADORA2A genotype (rs5751876)	No established target; the result is categorical	Predicts anxiety and sleep sensitivity to a given dose

Cadence: Blood pressure 4 weeks after any dose increase, then every 6–12 months; fasting glucose and HbA1c annually, or at 3 months where baseline glucose was impaired; sleep tracking reviewed whenever the daily total or the cut-off time changes.

Qualitative Assessment

Time to fall asleep, and whether it lengthens on higher-dose or later-dose days
Subjective sleep quality and morning refreshment
Afternoon energy dip, which signals late-day withdrawal rather than insufficient dosing
Resting jitteriness, hand tremor or palpitations
Anxiety level and irritability, particularly in the hours after the largest dose
Whether a missed dose produces headache, the clearest sign that dependence has been established
Perceived exertion during standard training sessions, on and off caffeine

Caffeine for Health & Longevity

Protocol

Benefits

Risk & Side Effects

Monitoring

Qualitative Assessment