How Does Caffeine Affect Your Body?

Medically Reviewed by Kathleen M. Zelman, RD, LD, MPH on May 02, 2023
4 min read

If you rely on a cup of coffee to jolt you awake in the morning or get you through an afternoon slump, you’re not alone. Four out of every five adults consume caffeine daily.

Your caffeine intake depends on the caffeine-containing products you prefer. A cup of tea only has 14 to 60 milligrams of caffeine in it, depending on the type of tea, while coffee has up to 200. You also take in caffeine if you enjoy energy drinks, colas, or chocolate products.

While caffeine usually isn’t dangerous in moderation, too much can negatively affect your health.

Most adults can safely consume about 400 milligrams of caffeine a day. That’s about what you’d get by drinking four to five average-sized cups of coffee.

That’s the typical safe amount, but there’s a lot of variation in how much caffeine people can tolerate. Signs that you’ve had too much include:

‌The more caffeine a food or drink item contains, the more likely you are to experience side effects.

Most people feel the maximum effect of caffeine about an hour after consumption. After four to six hours, your body will have burned off about half of what you took in.

Increased Alertness

Caffeine is a stimulant. In the brain, it blocks the effects of a chemical called adenosine, which makes you feel sleepy. You then feel more alert and energetic, which is why many people drink coffee or tea to stay awake.

Sleep Cycle Changes

Caffeine may keep you awake even if you don’t want it to. Levels of the hormone melatonin, which your body needs to fall asleep, decrease in the presence of caffeine. If you have trouble sleeping, you may want to limit your caffeine consumption in the afternoon and evening.

Feelings of Anxiety

The other downside is that caffeine may make you feel jittery or anxious, especially if you take in too much. If you have an existing anxiety or panic disorder, your doctor may advise you to avoid caffeine.

Increased Dopamine Levels

In other cases, caffeine can boost your mental well-being. It boosts the flow of dopamine, a brain chemical that makes you feel happy and engaged with the world around you. This may be why drinking a moderate amount of coffee has reduced some study participants’ risk of depression and suicide.

Positive Impact on Memory and Cognition

Caffeine may even help you to think and remember better. Study participants who drink coffee have performed better on tests. Also, older people who consume caffeine may be less likely to develop Alzheimer’s disease and other age-related memory issues.

Addiction and Withdrawal

There is a potential for addiction to caffeine. If you have headaches, feel depressed, or have trouble concentrating when you skip your usual intake, or if you feel like you can’t stop consuming caffeine despite side effects, you may be experiencing withdrawal.

Safe in Moderate Amounts

For the most part, caffeine is safe for your heart. There’s no evidence that it will increase your blood pressure or cause dangerous heart rhythm disturbances known as arrhythmias.

Caffeine from Energy Drinks Affect Your Heart Differently

Energy drinks may be the exception. If you get caffeine from a 32-ounce energy drink, you may be more likely to have abnormal heart rhythms and slightly high blood pressure six hours later.

Negative Effects for Those with Pre-Existing Heart Conditions

Less concentrated forms of caffeine may also temporarily increase your heart rate if you take in too much. This can be an unnerving sensation, but it’s normally not a concern unless you have pre-existing heart problems. If you do, it’s best to speak with a doctor about whether or not it’s safe to take in caffeine.

You might have heard some people say that coffee gives them heartburn. Caffeine does increase the amount of acid that your stomach produces.

Some scientists have connected this effect to the bitter taste of caffeine, so a more bitter-tasting food or drink would create more acid. 

If you have an acid reflux condition like gastroesophageal reflux disease (GERD), you may find that caffeine worsens your symptoms. Caffeine is also a diuretic, which means that it encourages your body to urinate more.

Too much caffeine can harm a developing baby, especially in the early stages of pregnancy. Even before you conceive, if you take in more than 400 milligrams of caffeine per day, there may be a negative effect. ‌

A fetus’s body breaks down caffeine more slowly because it lacks a specific chemical compound. That doesn’t mean you should avoid caffeine entirely during pregnancy, but obstetricians recommend limiting your intake to less than 200 milligrams per day.

If you have any further questions about caffeine intake and your body, consult a doctor.