Caffeine is a stimulant found in various foods, drinks, and other products. It’s commonly used to keep you awake and alert.
Studies have also shown that it’s safe for most people when consumed in low-to-moderate amounts.
However, high doses of caffeine may have unpleasant and even dangerous side effects.
Research has shown that your genes have a major influence on your tolerance to it. Some can consume much more caffeine than others without experiencing negative effects (5, 6).
What’s more, individuals who aren’t used to caffeine may experience symptoms after consuming what is typically considered a moderate dose (4, 7).
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).
Additionally, modest doses have been shown to cause rapid breathing and increase stress levels when consumed in one sitting (11, 12).
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.
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.
Studies have found that higher caffeine intake appears to increase the amount of time it takes to fall asleep. It may also decrease total sleeping time, especially in the elderly (13, 14).
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.
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.
3. Digestive Issues
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).
On the other hand, some studies suggest that caffeinated beverages may worsen gastroesophageal reflux disease (GERD) in some people. This seems to be especially true of coffee (23, 24, 25).
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.
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.
4. Muscle Breakdown
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 addition, there have been several reports of rhabdomyolysis related to excessive caffeine intake, although this is relatively rare (26, 27, 28, 29).
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.
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.