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Side Effects of Drinking Caffeine, According to Science

There's a lot to love about the stimulant found in coffee, tea, and cocoa, but it does come with some health precautions.

There's a lot to love about caffeine. First, because it's found in plants, it can actually be said to be all-natural, and those plants, including the ones used to make coffee, tea, and cocoa, are rich in sources of antioxidants and phytonutrients. Caffeine, itself, is a phytonutrient that can help reduce your risk of Parkinson's, dementia, and certain oral cancers, according to the University of Michigan Health Service.

One of caffeine's immediate physiological effects is to speed up your central nervous system, which can help you feel alert and focused. Another is that it increases levels of dopamine—which is one of those "feel-good" chemicals your body produces that improves your mood.

Consuming moderate amounts of caffeine (up to 400 milligrams per day, or the equivalent of four cups of coffee) has never been scientifically linked to long-term harm in healthy adults. However, that's not to say it doesn't have side effects.

For example, to the extent it can improve your mood, it can also be addictive. And while it can keep you focused, it can also leave you feeling overstimulated, according to registered dietitian Amy Goodson. Read on to learn more about the side effects of drinking caffeine, according to science. And don't forget to check out the 7 Healthiest Foods to Eat Right Now.

A temporary spike in blood pressure

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"Because caffeine is a stimulant, it increases your heart rate while it's in your system," says general practice physician, Leann Poston, MD. "At the same time, it causes the blood vessels to narrow, which increases the pressure the heart must exert to circulate blood." This sets you up for a temporary spike in blood pressure.

That said, high blood pressure is a serious condition that, over time, can put you at increased risk for heart disease, stroke, and kidney failure, among others. It's also associated with the most serious and even life-threatening presentation of the COVID-19 infection.

The good news is that the rise in blood pressure attributable to consuming caffeine is temporary, according to Sheldon G. Sheps, M.D., via Mayo Clinic. However, if you've been diagnosed with high blood pressure, it's not a bad idea to ask your doctor if you need to limit your intake of caffeinated beverages.

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woman with a headache

Most people associate headaches with caffeine-withdrawal. However, a 2004 study published in the scientific journal Neurology found that among people who suffer from chronic daily headaches, a condition in which headaches are suffered at least 15 times per month, a significant percentage had been heavy caffeine drinkers prior to the onset of their chronic daily headaches.

Similarly, among migraine sufferers, caffeine overuse has been associated with an increase in migraine frequency. If you get frequent headaches, you might want to consider cutting some of these headache-triggering foods out of your diet.


anxiety depression

"If you are already stressed or anxious, caffeine can make it worse," according to registered dietitian Elisa Bremner. In fact, as little as 100 milligrams of caffeine—the equivalent of one cup of coffee—may exacerbate pre-existing feelings of stress and anxiety. The scientific community offers several reasons for this. One is that the natural effects of caffeine as a stimulant cause a number of physical symptoms that we tend to associate with anxiety, including increased heart and breathing rates. Our bodies may experience those symptoms as anxiety.

In addition, caffeine stimulates the release of adrenaline, Dr. Poston tells Eat This, Not That! Adrenaline is a hormone that prepares us for "fight or flight" when we're faced with stressors. It does so by increasing alertness and raising heart rate, respiration, and blood flow. And because caffeine competes with a chemical called adenosine, which leads to feelings of fatigue and relaxation, the whole combination of all of these effects can lead to distinct feelings of nervousness, jitteriness, and anxiety.

Nevertheless, here is why you might want to consider drinking coffee before taking a nap.

Potential pregnancy complications

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Caffeine consumption has been linked to an increased risk of miscarriage, according to this paper published in 2020 in the scientific journal BMJ. The paper, which consisted of a meta-analysis of 48 existing studies addressing the link between caffeine consumption and negative pregnancy outcomes such as miscarriage, found that drinking coffee while pregnant is, in fact, associated with pregnancy loss.

The reason, according to OB/GYN, Jodie Horton, MD, may be linked to the fact that caffeine narrows blood vessels, which can diminish the transfer of nutrients to the fetus. It's worth pointing out, however, that these findings are at odds with the current recommendations from the American College of Obstetrics and Gynecology, which state that "moderate consumption" of caffeine does not present a miscarriage risk, according to OB/GYN Brittany Noel Robles, MD. Nevertheless, Dr. Robles recommends keeping caffeine to a minimum during pregnancy. (Related: Don't miss these 8 pregnancy myths, busted.)

Irritable bowel symptoms

Stomach pain

Caffeine's stimulant effect can help ensure that you'll keep your digestive system "moving." However, for some people, caffeine is too stimulating, which is to say, it can have a laxative effect.  In other words, caffeine can cause loose bowels and even diarrhea. Diarrhea is a major symptom of irritable bowel syndrome, according to Johns Hopkins Medicine's Health Blog.

If you're running to the bathroom every time you have caffeine, it may be worth talking to your doctor about whether you might be suffering from irritable bowel syndrome. Whether or not you are diagnosed, the discussion could lead to you deciding to limit your caffeine intake until you find a level at which consumption does not equal digestive distress.

For more on caffeine, here are 11 surprising ways that drinking tea can heal you.

Lauren Cahn
Lauren Cahn is a food, health, and culture writer whose work has appeared online and in print for Reader's Digest as well as Health Digest, Huffington Post, Taste of Home, and others. Read more about Lauren
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