Caffeine at a glance: schedule, half-life, and class
- 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
What does caffeine do?
Caffeine blocks adenosine, a brain chemical that builds up during waking hours and causes sleepiness. This increases alertness and reduces fatigue.[1]
As medicine, it is in some headache products, over-the-counter alertness tablets, and prescription caffeine citrate for breathing pauses in premature infants.
How much caffeine is too much?
Amounts vary widely by brand and serving. The FDA says up to 400 mg a day is generally safe for healthy adults; sensitivity differs from person to person.[1]
| Source | Typical caffeine | Notes |
|---|---|---|
| Brewed coffee, 8 oz | 80 to 100 mg | Coffee shop sizes often contain 200 to 400 mg |
| Black or green tea, 8 oz | 30 to 50 mg | |
| Cola, 12 oz | 30 to 40 mg | |
| Energy drink, 8 to 16 oz | 40 to 300 mg | Some contain more; check the label |
| Alertness tablet (NoDoz, Vivarin) | 200 mg | Do not exceed the label |
| Pure caffeine powder, 1 teaspoon | About 3,200 mg | A potentially lethal amount |
Caffeine side effects and risks
Common side effects of too much caffeine include jitteriness, anxiety, insomnia, fast heartbeat, upset stomach, and frequent urination.[1]
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.[2]
Sensitive groups: pregnant people, children and teens, and people with heart rhythm problems, anxiety disorders, or certain medicines should limit caffeine.
Caffeine interactions: what not to mix with it
Choose what it would be combined with:
| 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 |
This is not a full list. A pharmacist can check a complete medication list, including supplements and over-the-counter products.[1]
Caffeine overdose: signs and what to 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.[2]
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.[3]
- 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.
Reducing intake by a cup or so every few days avoids most withdrawal symptoms.
How do you buy caffeine safely in the US?
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.[2]
- Ask a pharmacist: a pharmacist can check whether it is safe with your other medicines and health conditions.
Can you buy caffeine online?
Caffeine products are widely sold online. Avoid pure or highly concentrated powders and liquids, which are easy to overdose on.
The FDA's BeSafeRx tool explains how to recognize a safe online pharmacy.[4]
Caffeine legal status in other countries
| 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 |
Laws change and differ in detail. Check current rules before carrying any controlled medicine across a border.[5]
Common 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.
