There's no one set of rules about ideal coffee intake; the right dose varies by individual, says Robert Graham, MD, the co-founder of FRESH Med, an integrative health practice in New York City. As long as the body can tolerate it without negative side effects, you should be fine with drinking around three to five, eight-ounce cups of coffee a day. (Each cup has about 100mg of caffeine per serving.) Most people, however, drink a lot less coffee.
It's a good guideline. But no matter how much (or how little) you've consumed, if you experience side effects — such as feeling anxious, shaky, or wired — it's a sign that you've overdone the caffeine. "Most people would agree that if you drink a lot of coffee, you might feel jittery, irritable or perhaps even nauseous," Elina Hyppönen, PhD, author of the study, previously told ScienceDaily. "That's because caffeine helps your body work faster and harder, but it's also likely to suggest that you may have reached your limit for the time being."
When you hit that point varies. Some may never really reach it, even after six cups. Others will start feeling off after just one mug.
One reason for that is because some people metabolize caffeine more slowly than others, according to research from the University of Toronto. The study found that caffeine only negatively affects people who are "slow caffeine metabolizers." Among that group, drinking just four or more cups a day was linked to a 36% higher chance of heart attacks. But "fast caffeine metabolizers" had no increased risk. In fact, for them, drinking three cups a day seemed to improve their heart health.
The bottom line: You have to listen to your body. Start to gauge how you feel after each cup or half cup. If you ever start to feel jittery, anxious, irritable, or shaky, that's a sign that you've exceeded your limit and should cut back. If you keep crossing that line over and over, you're putting your health at risk.
If you're one of those people who are never phased by caffeine, research indicates that it's still a good idea to try to stick to six cups or fewer a day. That should still be plenty to wake you up — without straining your heart.