As most coffee addicts know, there’s a fine line between pleasantly energized on caffeine, and too pumped up to think straight. Coffee is a ritual that’s repeated throughout the day: it keeps you awake, focused, and sharp. Caffeine hits quickly—it takes just 20 minutes—but go too far and you’ll get all-over jitters, heart palpitations, or even a full blown panic attack as the caffeine juggernaut charges through the system. For Bryan Jackson, the co-owner of Brooklyn Coffee in East London, drinking too much coffee is part of the job. After years in the trade he knows his caffeine limit, but every now and again it goes wrong—as was the case the last time he was trying out a new coffee for his shop. “I nearly died,” says Jackson—it’s a funny story when he tells it, but it’s clear the experience has left scars. 

It happened after Jackson had spent two hours tasting the new espresso variations at the shop. “I was starting to feel the effects, but against my better judgement I decided to meet a friend for wine, hoping that might bring me back down. But 30 minutes in I felt hot and dehydrated, and I was shaking.” Jackson went across the street to buy a banana—maybe some food would help? But as he waited to pay, he felt increasingly nauseous. “I didn't think I was going to be able to hold it.” When it was finally his turn to pay, the cashier took an interest in his tattoos, oblivious to Jackson’s dire state. “Finally, after the transaction was completed, I quickly devoured my banana and made it back across the street. But then, with one hand on the door of the wine bar, I felt it coming.” Jackson ran around the corner and vomited—and when he took the train home he had to get off at every stop to vomit some more. Yep, he laughs, he shat his pants too. “And of course, I couldn't sleep all night.”

As with wine, baristas will often spit out coffee during tastings, but Jackson says it’s sometimes necessary to swallow “to get a full understanding of the coffee as a whole.” So Jackson’s rough day can be viewed a sign of dedication to his job. 

Coffee overdose stories often feature all-nighters and deadlines, as was the case for Olaolu when he worked as a night editor. The coffee did the trick to keep him awake, he says, but the problems came the next night at work after not being able to sleep all day: ”I began to feel dizzy. I couldn't control myself: sleep was the only option.” After snoozing through an entire shift, Olaolu never drank coffee again. Although as Georgia learned the hard way when she was 17, quitting coffee cold turkey is not a good idea. Georgia went from 10 cups a day to zero: “I had a massive panic attack, where I burst into tears and told my dad I was going to fail my exams. Then I threw up.” After spending the whole weekend shaking, Georgia saw the light: she started drinking coffee again.

Too much caffeine can actually be lethal—but you’d need to drink about 100 cups of regular coffee to hit a danger zone. Moderate caffeine intake is fine as part of a healthy diet—the US Food and Drug Administration recommends no more than 400mg per day. “Depending on the strength of the coffee, 400mg is between three and five eight ounce cups,” says Caroline West Passerrello, a spokesperson for the Academy of Nutrition and Dietetics. But if you’re drinking coffee from those big travel mugs, you could hit the recommended caffeine limit after just two—not to mention how the trend of highly caffeinated coffees blows these recommendations out of the water. 

Routinely overdoing it could make you less sensitive to caffeine, meaning you’d need more to get that same kick. But the science on whether or not it coffee should be avoided is mixed: some studies say a little caffeine is good for the heart, whereas others say it’s not. “I think the problem with the studies is that everybody reacts differently to caffeine,” says West Passerrello. Anecdotal evidence backs this up: some people get heart palpitations after half a cup, but others drink ten coffees day and feel just fine. This is why you should listen to your body—those shakes will tell you when it’s time to put down that coffee mug. Says West Passerrello: “Whatever limit your body sets for you, you don’t want to habitually keep going over it.”