Short answer: Caffeine is the world's most widely used psychoactive substance, found in coffee, tea, soda, energy drinks, and over-the-counter alertness pills. For most healthy adults, up to 400 mg a day is not associated with dangerous effects. Concentrated powders and large amounts taken quickly can cause seizures, heart rhythm problems, and death.
Key takeaways
- What it is: Caffeine is a methylxanthine stimulant and not a federally controlled substance.
- Main uses: Fatigue; apnea in premature infants.
- Timing: effects start in 15 to 45 minutes and last 3 to 6 hours.
- Most dangerous mix: Alcohol, which can cause masked drunkenness, heavier drinking.
- Naloxone: it does not reverse this drug, but give it whenever opioids could be involved.
- Dependence risk: Low to moderate.
- Legal route: sold without a prescription; follow the Drug Facts label.
What is caffeine?
Caffeine blocks adenosine, a brain chemical that builds up during waking hours and causes sleepiness. This increases alertness and reduces fatigue.
As medicine, it is in some headache products, over-the-counter alertness tablets, and prescription caffeine citrate for breathing pauses in premature infants.
Quick facts
| Fact | Caffeine |
|---|---|
| Chemical name | 1,3,7-trimethylxanthine |
| Drug class | Methylxanthine stimulant; adenosine receptor antagonist |
| US legal status | Not controlled; food ingredient and OTC drug |
| Common sources | Coffee, tea, soda, energy drinks, chocolate, pills, pre-workout powders |
| FDA guide for adults | Up to 400 mg a day is generally not associated with dangerous effects |
| Pregnancy | Experts advise under 200 mg a day |
| Onset | 15 to 45 minutes; peak about 1 hour |
| Half-life | About 3 to 7 hours |
How long does caffeine last?
- Starts working: 15 to 45 minutes
- Effects last: 3 to 6 hours
- Half-life: About 3 to 7 hours
Its half-life is about 3 to 7 hours, so half a dose taken at 3 p.m. may still be active at bedtime.
Timing varies with the dose, the form, how it is taken, liver and kidney function, age, and other drugs taken at the same time.
What are the biggest risks of caffeine?
Common side effects of too much caffeine include jitteriness, anxiety, insomnia, fast heartbeat, upset stomach, and frequent urination.
Concentrated products: pure and highly concentrated powders and liquids make it easy to take a toxic dose. The FDA considers bulk pure caffeine sold directly to consumers unlawful.
Sensitive groups: pregnant people, children and teens, and people with heart rhythm problems, anxiety disorders, or certain medicines should limit caffeine.
What should you never mix with caffeine?
| Combined with | What happens | Guidance |
|---|---|---|
| Fluvoxamine, ciprofloxacin, oral contraceptives | Slower caffeine clearance; stronger effects | Reduce caffeine |
| Other stimulants (ADHD medicines, pseudoephedrine) | Jitteriness, fast heartbeat, insomnia | Limit caffeine |
| Alcohol | Masked drunkenness, heavier drinking | Avoid mixing |
| Lithium | Changes in caffeine intake alter lithium levels | Keep intake consistent |
| Theophylline | Additive toxicity | Limit |
Check any combination with the DoseUses combination checker or ask a pharmacist.
What are the signs of a caffeine overdose, and what should you do?
Caffeine toxicity causes vomiting, agitation, tremor, a very fast or irregular heartbeat, low potassium, and seizures. Deaths have occurred, mostly with powders or pills taken in large amounts.
If someone may be overdosing
- Call 911 for chest pain, a seizure, trouble breathing, signs of a stroke (face drooping, weakness on one side, slurred speech), a very high body temperature, or if the person collapses.
- Move them somewhere cool and calm, loosen tight clothing, and cool the skin with water. Do not try to hold them down.
- If they become unresponsive or their breathing slows, give naloxone. Street stimulants are often contaminated with fentanyl.
- For advice when the person is awake and stable, call Poison Help at 1-800-222-1222.
What is caffeine withdrawal like?
Regular caffeine users become physically dependent. Withdrawal is recognized in the DSM-5 and is common but not dangerous.
- 12 to 24 hoursSymptoms beginHeadache, tiredness, and sleepiness.
- Days 1 to 2PeakHeadache, low mood, irritability, poor concentration, and flu-like feelings.
- 2 to 9 daysResolvesSymptoms fade; cutting down gradually avoids most of them.
See the withdrawal timelines and the full caffeine withdrawal guide.
Is caffeine legal?
Caffeine is legal and unregulated as a drug for adults in the United States. It is a food ingredient and an over-the-counter drug in alertness tablets.
- Labels: FDA rules do not require caffeine amounts on all food labels, but many energy drinks list them.
- Powders: the FDA has said that highly concentrated or pure caffeine sold in bulk to consumers is unlawful.
- Ask a pharmacist: a pharmacist can check whether it is safe with your other medicines and health conditions.
Outside the United States
| Country | Status |
|---|---|
| European Union and United Kingdom | Drinks with over 150 mg per liter must carry a high-caffeine warning |
| International | Not under UN control |
Frequently asked questions about caffeine
How much caffeine is safe per day?
Up to 400 mg a day for most healthy adults, according to the FDA; less in pregnancy and for teens.
How long does caffeine stay in your system?
Its half-life is about 3 to 7 hours, so half a dose taken at 3 p.m. may still be active at bedtime.
Can you overdose on caffeine?
Yes. Large amounts, especially from powders or pills, can cause seizures, heart rhythm problems, and death.
Why do I get headaches when I skip coffee?
Caffeine withdrawal causes headaches within 12 to 24 hours in regular users.
Is caffeine addictive?
It causes physical dependence and withdrawal, but most people can cut down without serious problems.
Sources
- Spilling the Beans: How Much Caffeine is Too Much?, US Food and Drug Administration.
- Pure and Highly Concentrated Caffeine, US Food and Drug Administration.
- Caffeine, MedlinePlus, US National Library of Medicine.
- BeSafeRx: Your Source for Online Pharmacy Information, US Food and Drug Administration.
- International drug control conventions, United Nations Office on Drugs and Crime.
This guide summarizes the sourced DoseUses Caffeine profile, written from FDA labels, DEA rules, and federal health agencies. It is educational and is not medical advice. Last updated October 5, 2026.
