Health Topic

Caffeine

Many people enjoy a cup of coffee or tea to give them a boost. But is it safe? Learn more about caffeine.

Reviewed Reviewed by a registered pharmacistGeneral information

Important: This page is general information, not medical advice. It does not recommend or prescribe any treatment. Always consult your doctor or pharmacist before starting, stopping or changing any medicine.

About Caffeine

Caffeine is a bitter substance that occurs naturally in more than 60 plants including:

  • Coffee beans
  • Tea leaves
  • Kola nuts, which are used to flavor soft drink colas
  • Cacao pods, which are used to make chocolate products

There is also synthetic (man-made) caffeine, which is added to some medicines, foods, and drinks. For example, some pain relievers, cold medicines, and over-the-counter medicines for alertness contain synthetic caffeine. So do energy drinks and "energy-boosting" gums and snacks.

Most people consume caffeine from drinks. The amount of caffeine in different drinks can vary a lot, but it is generally:

  • An 8-ounce cup of coffee: 95-200 mg
  • A 12-ounce can of cola: 35-45 mg
  • An 8-ounce energy drink: 70-100 mg
  • An 8-ounce cup of tea: 14-60 mg

Caffeine has many effects on your body's metabolism. It:

  • Stimulates your central nervous system, which can make you feel more awake and give you a boost of energy
  • Is a diuretic, meaning that it helps your body get rid of extra salt and water by urinating more
  • Increases the release of acid in your stomach, sometimes leading to an upset stomach or heartburn
  • May interfere with the absorption of calcium in the body
  • Increases your blood pressure

Within one hour of eating or drinking caffeine, it reaches its peak level in your blood. You may continue to feel the effects of caffeine for four to six hours.

For most people, it is not harmful to consume up to 400mg of caffeine a day. If you do eat or drink too much caffeine, it can cause health problems, such as:

  • Restlessness and shakiness
  • Insomnia
  • Headaches
  • Dizziness
  • Fast heart rate
  • Dehydration
  • Anxiety
  • Dependency, so you need to take more of it to get the same results

Some people are more sensitive to the effects of caffeine than others.

Energy drinks are beverages that have added caffeine. The amount of caffeine in energy drinks can vary widely, and sometimes the labels on the drinks do not give you the actual amount of caffeine in them. Energy drinks may also contain sugars, vitamins, herbs, and supplements.

Companies that make energy drinks claim that the drinks can increase alertness and improve physical and mental performance. This has helped make the drinks popular with American teens and young adults. There's limited data showing that energy drinks might temporarily improve alertness and physical endurance. There is not enough evidence to show that they enhance strength or power. But what we do know is that energy drinks can be dangerous because they have large amounts of caffeine. And since they have lots of sugar, they can contribute to weight gain and worsen diabetes.

Sometimes young people mix their energy drinks with alcohol. It is dangerous to combine alcohol and caffeine. Caffeine can interfere with your ability to recognize how drunk you are, which can lead you to drink more. This also makes you more likely to make bad decisions.

You should check with your health care provider about whether you should limit or avoid caffeine if you:

  • Are pregnant, since caffeine passes through the placenta to your baby.
  • Are breastfeeding, since a small amount of caffeine that you consume is passed along to your baby.
  • Have sleep disorders, including insomnia.
  • Have migraines or other chronic headaches.
  • Have anxiety.
  • Have GERD or ulcers.
  • Have arrhythmia (a problem with the rate or rhythm of your heartbeat).
  • Have high blood pressure.
  • Take certain medicines or supplements, including stimulants, certain antibiotics, asthma medicines, and heart medicines. Check with your health care provider about whether there might be interactions between caffeine and any medicines and supplements that you take.
  • Are a child or teen. Neither should have as much caffeine as adults. Children can be especially sensitive to the effects of caffeine.

If you have been consuming caffeine on a regular basis and then suddenly stop, you may have caffeine withdrawal. Symptoms can include:

  • Headaches
  • Drowsiness
  • Irritability
  • Nausea
  • Trouble concentrating

These symptoms usually go away after a couple of days.

It sits within Food and Nutrition.

When to speak to someone. Reading about a topic cannot tell you whether it applies to you. Speak to a doctor or pharmacist if symptoms are severe, sudden, getting worse, or simply not going away — and seek emergency care immediately for chest pain, breathing difficulty, sudden weakness or confusion, or a serious allergic reaction.

Research on Caffeine

579 studies areregistered for this condition on ClinicalTrials.gov, the U.S. National Library of Medicine's public registry. A selection is below — the links go straight to the registry entry, not to a summary of ours.

What a registered study does and does not mean. Registration means a study exists and has been declared publicly. It says nothing about whether the treatment being tested works, is safe, or is available to you. Many studies find no benefit — that is what research is for. Never seek out an experimental treatment on the strength of a registry entry alone.

Research currently under way

Research already completed

Study data from ClinicalTrials.gov, a service of the U.S. National Library of Medicine. Retrieved 2026-07-30. Registry entries are supplied by study sponsors and investigators; ClinicalTrials.gov does not verify their scientific validity. PocketsInfo is not affiliated with, and not endorsed by, the NLM or the NIH.

Where to read more

Rather than paraphrase, we point you to the primary sources. Every link below is to a government health agency, a national institute or a recognised medical body — grouped by what you are trying to find out.

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Related Issues

Clinical Trials

Medical subject headings

Clinicians and researchers index this subject under Caffeine. These are the terms to use when searching medical literature.

This page is built from the MedlinePlus health topic record for Caffeine, a service of the U.S. National Library of Medicine (NLM), National Institutes of Health. Information is from MedlinePlus.gov. PocketsInfo is not affiliated with, and is not endorsed by, the NLM or the NIH. Retrieved 30 July 2026.