What too much caffeine can do

According to Marilyn Cornelis, an associate professor of preventive medicine at Northwestern University Feinberg School of Medicine, coffee has thousands of chemical compounds, a number of which can influence your health.

In fact, studies have suggested that coffee drinkers live longer and have lower risks of Type 2 diabetes, Parkinson's disease, cardiovascular conditions, and some cancers.

But if there's a risk in coffee consumption, it comes from the caffeine, Cornelis said.

Having too much caffeine can lead to a racing heart, jitteriness, anxiousness, nausea, or trouble sleeping, according to Jennifer Temple, a professor of exercise and nutrition sciences at the University at Buffalo.

Too much caffeine can also lead to headaches, acid reflux, and even tremors or vomiting at high enough doses, said Adrienne Hughes, a medical toxicologist and assistant professor of emergency medicine at Oregon Health and Science University.

But generally speaking, "most people are kind of well-tuned with their response to caffeine," Cornelis said. And when they start to experience mild symptoms of too much caffeine, they slow down.

As a result, it's not often that people experience dangerous side effects from drinking coffee, Hughes said. Typically, caffeine overdoses result from more concentrated forms like powders or supplements, and in most cases, a person would have to consume at least 10,000 milligrams of caffeine — which is equivalent to around 50 to 100 cups of coffee — for it to be fatal.

Still, caffeine can cause a short-term increase in blood pressure and heart rate, especially if you don't regularly consume it, Hughes added. This typically isn't harmful, as research has found drinking coffee isn't associated with higher blood pressure or abnormal heart rhythm risk in the long run.

But, if you're someone who is prone to abnormal heart rhythms, or if you notice palpitations after having caffeine, it's possible you could be more sensitive to it than others. In this case, you shouldn't consume more than you're used to or consume large doses of it from sources like supplements or energy shots, Hughes said.

In addition, according to Rob van Dam, a professor of exercise and nutrition sciences at the Milken Institute School of Public Health at George Washington University, having too much caffeine while pregnant has been associated with an increased risk of miscarriage.

How much coffee is too much?

According to FDA, most adults can safely consume 400 milligrams of caffeine per day, which is roughly equivalent to four eight-ounce cups of brewed coffee or six espresso shots. However, if you're pregnant, the American College of Obstetricians and Gynecologists recommends you consume no more than 200 milligrams of coffee per day.

Van Dam said he believes the 400-milligram guideline is reasonable for most adults and fits with what the research says you should consume to reap health benefits while avoiding negative side effects. According to him, two to four cups of coffee a day is "kind of a sweet spot."

However, van Dam noted that people break down caffeine at different rates, so 400 milligrams may feel like too much to some while others won't have any problems.

According to Cornelis, it can take anywhere from two to 10 hours to clear half a dose of caffeine from your blood, depending on your genetics. If you're someone who's on the longer end of that spectrum, drinking espresso midway through the afternoon might lead to problems sleeping, while if you're someone who metabolizes caffeine faster, it might not cause any issues.

Smoking tobacco can also increase the speed of your caffeine metabolism significantly, which is why people who smoke might feel the need to consume more caffeine to stay awake. In addition, being pregnant or taking oral contraceptives can slow down that metabolism, van Dam said.

Ultimately "you just kind of have to listen to your body," Temple said. "If you're starting to feel nauseous or jittery or anxious, maybe cut back," she added. "If it's affecting your sleep, cut back." (Callahan, New York Times, 9/26)

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