Coffee and tea are incredibly healthy beverages.
Studies have also shown that it’s safe for most people when consumed in low-to-moderate amounts (4).
However, high doses of caffeine may have unpleasant and even dangerous side effects.
Here are 9 side effects of too much caffeine.
Caffeine is known to increase alertness.
It works by blocking the effects of adenosine, a brain chemical that makes you feel tired. At the same time, it triggers the release of adrenaline, the “fight-or-flight” hormone associated with increased energy (8).
However, at higher doses, these effects may become more pronounced, leading to anxiety and nervousness.
In fact, caffeine-induced anxiety disorder is one of four caffeine-related syndromes listed in the Diagnostic and Statistical Manual of Mental Disorders (DSM), which is published by the American Psychiatric Association.
Extremely high daily intakes of 1,000 mg or more per day have been reported to cause nervousness, jitteriness and similar symptoms in most people, whereas even a moderate intake may lead to similar effects in caffeine-sensitive individuals (9, 10).
One study in 25 healthy men found that those who ingested approximately 300 mg of caffeine experienced more than double the stress of those who took a placebo.
Interestingly, stress levels were similar between regular and less frequent caffeine consumers, suggesting the compound may have the same effect on stress levels regardless of whether you drink it habitually (12).
Nevertheless, these results are preliminary.
Coffee’s caffeine content is highly variable. For reference, a large (“grande”) coffee at Starbucks contains about 330 mg of caffeine.
If you notice that you often feel nervous or jittery, it might be a good idea to look at your caffeine intake and cut it back.
Summary: Although low-to-moderate doses of caffeine can increase alertness, larger amounts may lead to anxiety or edginess. Monitor your own response in order to determine how much you can tolerate.
Caffeine’s ability to help people stay awake is one of its most prized qualities.
On the other hand, too much caffeine can make it difficult to get enough restorative sleep.
By contrast, low or moderate amounts of caffeine don’t seem to affect sleep very much in people considered “good sleepers,” or even those with self-reported insomnia (15).
You may not realize that too much caffeine is interfering with your sleep if you underestimate the amount of caffeine you’re taking in.
Although coffee and tea are the most concentrated sources of caffeine, it is also found in soda, cocoa, energy drinks and several types of medication.
For example, an energy shot may contain up to 350 mg of caffeine, while some energy drinks provide as much as a whopping 500 mg per can (16).
Importantly, the amount of caffeine you can consume without affecting your sleep will depend on your genetics and other factors.
In addition, caffeine consumed later in the day may interfere with sleep because its effects can take several hours to wear off.
Research has shown that while caffeine remains in your system for an average of five hours, the time period may range from one and a half hours to nine hours, depending on the individual (17).
One study investigated how the timing of caffeine ingestion affects sleep. Researchers gave 12 healthy adults 400 mg of caffeine either six hours before bedtime, three hours before bedtime or immediately prior to bedtime.
Both the time it took all three groups to fall asleep and the time they spent awake at night increased significantly (18).
These results suggest that it’s important to pay attention to both the amount and timing of caffeine to optimize your sleep.
Summary: Caffeine can help you stay awake during the day, but it may negatively impact your sleep quality and quantity. Cut off your caffeine consumption by the early afternoon to avoid sleeping problems.
Many people find that a morning cup of coffee helps get their bowels moving.
Coffee’s laxative effect has been attributed to the release of gastrin, a hormone the stomach produces that speeds up activity in the colon. What’s more, decaffeinated coffee has been shown to produce a similar response (19, 20, 21).
However, caffeine itself also seems to stimulate bowel movements by increasing peristalsis, the contractions that move food through your digestive tract (21).
Given this effect, it’s not surprising that large doses of caffeine may lead to loose stools or even diarrhea in some people.
Although for many years coffee was believed to cause stomach ulcers, a large study of more than 8,000 people didn’t find any link between the two (22).
In a small study, when five healthy adults drank caffeinated water, they experienced a relaxation of the muscle that keeps stomach contents from moving up into the throat — the hallmark of GERD (25).
Since coffee can have major effects on digestive function, you may want to cut back on the amount you drink or switch to tea if you experience any issues.
Summary: Although small to moderate amounts of coffee can improve gut motility, larger dosages may lead to loose stools or GERD. Reducing your coffee intake or switching to tea may be beneficial.
Rhabdomyolysis is a very serious condition in which damaged muscle fibers enter the bloodstream, leading to kidney failure and other problems.
Common causes of rhabdomyolysis include trauma, infection, drug abuse, muscle strain and bites from poisonous snakes or insects.
In one case, a woman developed nausea, vomiting and dark urine after drinking 32 ounces (1 liter) of coffee containing roughly 565 mg of caffeine. Fortunately, she recovered after being treated with medication and fluids (29).
Importantly, this is a large dosage of caffeine to consume within a short period of time, especially for someone who isn’t used to it or is highly sensitive to its effects.
In order to reduce the risk of rhabdomyolysis, it’s best to limit your intake to about 250 mg of caffeine per day, unless you’re used to consuming more.
Summary: People may develop rhabdomyolysis, or the breakdown of damaged muscle, after they ingest large amounts of caffeine. Limit your intake to 250 mg per day if you’re uncertain of your tolerance.
Despite all of caffeine’s health benefits, there’s no denying that it may become habit-forming.
A detailed review suggests that although caffeine triggers certain brain chemicals similarly to the way cocaine and amphetamines do, it does not cause classic addiction the way these drugs do (30).
However, it may lead to psychological or physical dependency, especially at high dosages.
In one study, 16 people who typically consumed high, moderate or no caffeine took part in a word test after going without caffeine overnight. Only high caffeine users showed a bias for caffeine-related words and had strong caffeine cravings (31).
Additionally, the frequency of caffeine intake seems to play a role in dependency.
In another study, 213 caffeine users completed questionnaires after going 16 hours without consuming it. Daily users had greater increases in headaches, fatigue and other withdrawal symptoms than non-daily users (32).
Even though the compound does not seem to cause true addiction, if you regularly drink a lot of coffee or other caffeinated beverages, there’s a very good chance you may become dependent on its effects.
Summary: Going without caffeine for several hours may lead to psychological or physical withdrawal symptoms in those who consume large amounts on a daily basis.
Overall, caffeine doesn’t seem to increase the risk of heart disease or stroke in most people.
Elevated blood pressure is a risk factor for heart attack and stroke because it may damage arteries over time, restricting the flow of blood to your heart and brain.
Fortunately, caffeine’s effect on blood pressure seems to be temporary. Also, it seems to have the strongest impact on people who aren’t used to consuming it.
Therefore, paying attention to the dosage and timing of caffeine is important, especially if you already have high blood pressure.
Summary: Caffeine seems to raise blood pressure when consumed at high doses or prior to exercise, as well as in people who rarely consume it. But this effect may only be temporary, so it’s best to monitor your response.
The stimulatory effects of high caffeine intake may cause your heart to beat faster.
It may also lead to altered heartbeat rhythm, called atrial fibrillation, which has been reported in young people who consumed energy drinks containing extremely high doses of caffeine (39).
In one case study, a woman who took a massive dose of caffeine powder and tablets in an attempted suicide developed a very rapid heart rate, kidney failure and other serious health issues (40).
However, this effect doesn’t seem to occur in everyone. Indeed, even some people with heart problems may be able to tolerate large amounts of caffeine without any adverse effects.
In one controlled study, when 51 heart failure patients consumed 100 mg of caffeine per hour for five hours, their heart rates and rhythms remained normal (41).
Regardless of the mixed study results, if you notice any changes in your heart rate or rhythm after drinking caffeinated beverages, consider decreasing your intake.
Summary: Large doses of caffeine may increase heart rate or rhythm in some people. These effects appear to vary greatly from person to person. If you feel them, consider reducing your intake.
Coffee, tea and other caffeinated beverages are known to boost energy levels.
However, they can also have the opposite effect by leading to rebound fatigue after the caffeine leaves your system.
One review of 41 studies found that although caffeinated energy drinks increased alertness and improved mood for several hours, participants were often more tired than usual the following day (42).
Of course, if you continue to drink lots of caffeine throughout the day, you can avoid the rebound effect. On the other hand, this may affect your ability to sleep.
To maximize caffeine’s benefits on energy and avoid rebound fatigue, consume it in moderate rather than high doses.
Summary: Although caffeine provides energy, it can indirectly lead to fatigue when its effects wear off. Aim for moderate caffeine intake to help minimize rebound fatigue.
Increased urination is a common side effect of high caffeine intake due to the compound’s stimulatory effects on the bladder.
You may have noticed that you need to urinate frequently when you drink more coffee or tea than usual.
In one study, 12 young to middle-aged people with overactive bladders who consumed 2 mg of caffeine per pound (4.5 mg per kilogram) of body weight daily experienced significant increases in urinary frequency and urgency (44).
For someone weighing 150 pounds (68 kg), this would equate to about 300 mg of caffeine per day.
In addition, high intake may increase the likelihood of developing incontinence in people with healthy bladders.
One large study looked at the effects of high caffeine intake on incontinence in more than 65,000 women without incontinence.
Those who consumed more than 450 mg daily had a significantly increased risk of incontinence, compared to those who consumed less than 150 mg per day (45).
If you drink a lot of caffeinated beverages and feel that your urination is more frequent or urgent than it should be, it may be a good idea to cut back on your intake to see if your symptoms improve.
Summary: High caffeine intake has been linked to increased urinary frequency and urgency in several studies. Reducing your intake may improve these symptoms.
Light-to-moderate caffeine intake seems to provide impressive health benefits in many people.
On the other hand, very high dosages may lead to side effects that interfere with day-to-day living and might even cause serious health issues.
Although responses vary from person to person, the effects of high intake demonstrate that more isn’t necessarily better.
To get the benefits of caffeine without undesirable effects, conduct an honest assessment of your sleep, energy levels and other factors that might be affected, and reduce your intake if needed.