Go ahead, have another cup.
An unprecedented scientific review on caffeine safety confirms that drinking up to four cups of coffee daily, or about 400 milligrams of caffeine, is “not associated with overt, adverse effects” in healthy adults. (Pregnant woman and minors should reduce their intake below that amount, according to the report.) 
The review was conducted by the International Life Sciences Institute (ILSI), and is the most extensive of its kind to date. Scientists scoured data from more than 700 independent studies related to various human health effects and caffeine.
Coffee in Context
It is important to note that the 400 milligram amount is a “guideline.” Tolerance of caffeine (known technically as 1,3,7-trimethylxanthine) is highly variable, and affected by factors ranging from metabolism to heredity. Luckily, our bodies can self-regulate consumption by sending signals to our brain when it’s time to cut off the caffeine.
What does 400 milligrams mean in terms of what you are actually drinking?
First, know that caffeine levels can vary from beverage to beverage, and even from the beans on one coffee tree to another. One yardstick is that a typical 8 oz. cup of drip coffee has about 100 milligrams of caffeine.
Most Americans consume about 165 milligrams of caffeine daily, according to 2014 research. This comes to less than 2 cups of coffee – significantly below the 3-5 cups suggested by the U.S. Dietary Guidelines. Yet 66% of coffee drinkers say that they limit their coffee consumption due to concerns about caffeine, according to the 2017 National Coffee Drinking Trends report.
Caffeine, of course, is just one aspect of coffee. Drinking coffee has been associated with many potential health benefits, from preventing liver disease to extending longevity. In fact, last year, IARC concluded that coffee may even help prevent certain types of cancer.
And although many of coffee’s positive effects are linked directly to caffeine, decaffeinated coffee always is an option. (In fact, just smelling roasted coffee can give your energy levels a boost, according to research.)
Ultimately, your coffee choices are about you, and your individual tastes, preferences and tolerances. Only you can decide what’s right for you, whether you prefer a double espresso or a decaf cappuccino. If you’ve got concerns about caffeine consumption you should listen to your body, consult reliable research available online, and consult with a medical professional. And of course, remember that coffee may not be your only dietary source of caffeine.
As with most things, whether you like your coffee black, sweet, or light, coffee is best taken along with a serving of common sense.
 NCA contributed to the work of ILSI, but did not participate in the work itself, which consisted of reviewing work done previously by other scientists to reach overall conclusions about a body of preexisting research.