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What is caffeine, and is it good or bad for health?

Caffeine is a natural stimulant consumed throughout the world. This article reviews caffeine and its health effects, both good and bad.

This article is based on scientific evidence, written by experts, and fact-checked by experts.
We look at both sides of the argument and strive to be objective, unbiased, and honest.
What is caffeine, and is it good or bad for health?
Last updated on March 8, 2023, and last reviewed by an expert on October 23, 2021.

Each day, billions of people rely on caffeine to wake up, or to get through that night shift or an afternoon slump.

What is caffeine, and is it good or bad for health?

This natural stimulant is one of the most commonly used ingredients in the world.

Caffeine is often talked about for its negative effects on sleep and anxiety.

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However, studies also report that it has various health benefits.

This article examines the latest research on caffeine and your health.

In this article

What is caffeine?

Caffeine is a natural stimulant most commonly found in tea, coffee, and cacao plants.

It works by stimulating the brain and central nervous system, helping you stay alert and prevent the onset of tiredness.

Historians track the first brewed tea as far back as 2737 B.C.

Coffee was reportedly discovered many years later by an Ethiopian shepherd who noticed the extra energy it gave his goats.

Caffeinated soft drinks hit the market in the late 1800s and energy drinks soon followed.

Nowadays, 80% of the world’s population consumes a caffeinated product each day, and this number goes up to 90% for adults in North America.

Summary: Caffeine is a natural stimulant that’s widely consumed worldwide. It helps you stay awake and can stave off tiredness.

How caffeine works

Once consumed, caffeine is quickly absorbed from the gut into the bloodstream.

From there, it travels to the liver and is broken down into compounds that can affect the function of various organs.

That said, caffeine’s main effect is on the brain.

It functions by blocking the effects of adenosine, which is a neurotransmitter that relaxes the brain and makes you feel tired.

Normally, adenosine levels build up over the day, making you increasingly more tired and causing you to want to go to sleep.

Caffeine helps you stay awake by connecting to adenosine receptors in the brain without activating them. This blocks the effects of adenosine, leading to reduced tiredness.

It may also increase blood adrenaline levels and increase brain activity of the neurotransmitters dopamine and norepinephrine.

This combination further stimulates the brain and promotes a state of arousal, alertness, and focus. Because it affects your brain, caffeine is often referred to as a psychoactive drug.

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Additionally, caffeine tends to exert its effects quickly.

For instance, the amount found in one cup of coffee can take as little as 20 minutes to reach the bloodstream and about 1 hour to reach full effectiveness.

Summary: Caffeine’s main effect is on the brain. It stimulates the brain by blocking the effects of the neurotransmitter adenosine.

What foods and drinks contain caffeine?

Caffeine is naturally found in the seeds, nuts, or leaves of certain plants.

These natural sources are then harvested and processed to produce caffeinated foods and beverages.

Here are the amounts of caffeine expected per 8-ounce (240-mL) serving of some popular beverages:

Some foods also contain caffeine. For instance, 1 ounce (28 grams) of milk chocolate contains 1–15 mg, whereas 1 ounce of dark chocolate has 5–35 mg.

You can also find caffeine in some prescription or over-the-counter drugs like cold, allergy, and pain medications. It’s also a common ingredient in weight loss supplements.

Summary: Caffeine is most commonly found in coffee, tea, soft drinks, chocolate, and energy drinks.

Caffeine may improve mood and brain function

Caffeine can block the brain-signaling molecule adenosine.

This causes a relative increase in other signaling molecules, such as dopamine and norepinephrine.

This change in brain messaging is thought to benefit your mood and brain function.

One review reports that after participants ingested 37.5–450 mg of caffeine, they had improved alertness, short-term recall, and reaction time.

Suggested read: How much caffeine is in green tea?

In addition, a study linked drinking 2–3 cups of caffeinated coffee (providing about 200–300 mg caffeine) per day to a 45% lower risk of suicide.

Another study reported a 13% lower risk of depression in caffeine consumers.

When it comes to mood, more caffeine isn’t necessarily better.

A study found that the second cup of coffee produced no further benefits unless it was consumed at least 8 hours after the first cup.

Drinking between 3–5 cups of coffee per day or more than 3 cups of tea per day may also reduce the risk of brain diseases such as Alzheimer’s and Parkinson’s by 28–60%.

It’s important to note that coffee and tea contain other bioactive compounds (besides caffeine) that may also be beneficial.

Summary: Caffeine may improve mood, decrease the likelihood of depression, stimulate brain function, and protect against Alzheimer’s and Parkinson’s disease.

Caffeine may boost metabolism and fat burning

Because of its ability to stimulate the central nervous system, caffeine may increase metabolism by up to 11% and fat burning by up to 13%.

Practically speaking, consuming 300 mg of caffeine per day may allow you to burn an extra 79 calories daily.

This amount may seem small, but it’s similar to the calorie excess responsible for the average yearly weight gain of 2.2 pounds (1 kg) in Americans.

However, a 12-year study on caffeine and weight gain noted that the participants who drank the most coffee were, on average, only 0.8–1.1 pounds (0.4–0.5 kg) lighter at the end of the study.

Summary: Caffeine may boost metabolism and promote fat loss, but these effects are likely to remain small over the long term.

Caffeine may enhance exercise performance

When it comes to exercise, caffeine may increase the use of fat as fuel.

This is beneficial because it can help the glucose stored in muscles last longer, potentially delaying the time it takes your muscles to reach exhaustion.

Caffeine may also improve muscle contractions and increase tolerance to fatigue.

Researchers observed that doses of 2.3 mg per pound (5 mg per kg) of body weight improved endurance performance by up to 5% when consumed 1 hour before exercise.

Doses as low as 1.4 mg per pound (3 mg per kg) of body weight may be sufficient to reap the benefits.

Suggested read: Coffee and caffeine — How much should you drink?

Also, studies report similar benefits in team sports, high-intensity workouts, and resistance exercises.

Finally, it may also reduce perceived exertion during exercise by up to 5.6%, which can make workouts feel easier.

Summary: Consuming small amounts of caffeine about an hour before exercise is likely to improve exercise performance.

Caffeine may protect against heart disease and diabetes

Despite what you may have heard, caffeine doesn’t raise the risk of heart disease.

Evidence shows a 16–18% lower risk of heart disease in men and women who drink between 1–4 cups of coffee daily (providing approximately 100–400 mg of caffeine).

Other studies show that drinking 2–4 cups of coffee or green tea per day is linked to a 14–20% lower risk of stroke.

One thing to keep in mind is that caffeine may slightly raise blood pressure in some people. However, this effect is generally small (3–4 mmHg) and tends to fade for most individuals when they consume coffee regularly.

It may also protect against diabetes.

A review noted that those who drink the most coffee have up to a 29% lower risk of developing type 2 diabetes. Similarly, those who consume the most caffeine have up to a 30% lower risk.

The authors observed that the risk drops by 12–14% for every 200 mg of caffeine consumed.

Interestingly, consuming decaffeinated coffee was also linked to a 21% lower risk of diabetes. This indicates that other beneficial compounds in coffee can also protect against type 2 diabetes.

Summary: Caffeinated beverages like coffee and tea may reduce the risk of heart disease and type 2 diabetes, although this may depend on the individual.

Other health benefits of coffee

Coffee consumption is linked to several other health benefits:

Keep in mind that coffee also contains other substances that improve health. Some benefits listed above may be caused by substances other than caffeine.

Suggested read: Can coffee increase your metabolism and help you burn fat?

Summary: Drinking coffee may promote a healthy liver, skin, and digestive tract. It may also prolong life and help prevent several diseases.

Safety and side effects of caffeine

Caffeine consumption is generally considered safe, although habit-forming.

Some side effects linked to excess intake include anxiety, restlessness, tremors, irregular heartbeat, and trouble sleeping.

Too much caffeine may also promote headaches, migraines, and high blood pressure in some individuals.

In addition, caffeine can easily cross the placenta, which can increase the risk of miscarriage or low birth weight. Pregnant women should limit their intake.

Caffeine can also interact with some medications.

Individuals taking the muscle relaxant Zanaflex or the antidepressant Luvox should avoid caffeine because these drugs can increase their effects.

Summary: Caffeine can have negative side effects in some people, including anxiety, restlessness, and trouble sleeping.

Both the U.S. Department of Agriculture (USDA) and the European Food Safety Authority (EFSA) consider a daily intake of 400 mg of caffeine to be safe. This amounts to 2–4 cups of coffee per day.

That said, it’s worth noting that fatal overdoses have been reported with single doses of 500 mg of caffeine.

Therefore, it’s recommended to limit the amount of caffeine you consume at one time to 200 mg per dose.

According to the American College of Obstetricians and Gynecologists, pregnant women should limit their daily intake to 200 mg.

Summary: A caffeine intake of 200 mg per dose, and up to 400 mg per day, is generally considered safe. Pregnant women should limit their daily intake to 200 mg or less.


Caffeine isn’t as unhealthy as it was once believed.

Evidence shows that it may be just the opposite.

Therefore, it’s safe to consider your daily cup of coffee or tea as an enjoyable way to promote good health.

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