It's probably more than you think. 
Healthy caffeine intake
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A new study in the journal Food and Chemical Toxicology has updated 15 years of conflicting research on how much caffeine is actually safe for people to drink.

The researchers “conducted a systematic review of data on potential adverse effects of caffeine published from 2001 to June 2015,” studying in particular the effects of caffeine on cardiovascular and bone health, behavior, and development.

After pouring over nearly 5,000 articles, the team determined that a healthy adult is in the clear to drink 400 mg of caffeine everyday.

What does that mean for you? Well, if you’re a fan of a plain, regular brewed 8 oz. coffee, you can safely drink 4 cups everyday. If you like a quick boost of energy better, there are 64 mg of caffeine in one shot of espresso, which means that even six shots of that would technically be safe, but you’d probably end up a jittery mess (a typical latte has even more, at an average of 77 mg).

According to the FDA, the average person drinks around 300 mg of caffeine each day, so most people are well within the healthy range already. But this doesn’t mean you could hypothetically drink say, 12 cans of soda every day (which typically contain around 29 mg of caffeine), which carry their own set of serious health risks, and were recently linked to stroke and dementia.

The benefits of coffee have been widely studied: Last year, doctors at Southampton University found that drinking more coffee might help reverse liver damage done by alcohol. Participants in their study who drank two more cups of coffee than usual saw a 44 percent drop in their chances of developing liver cirrhosis. One 2015 article highlighted studies from the Harvard T.H. Chan School of Public Health that have shown that coffee can lower depression in some women, and protect against Type 2 diabetes and cardiovascular disease.

There you have it, science has given you the all clear: Go get that second (or third) cup.