How Dangerous Is it to Black Out From Alcohol?

The Scenario: Your friend drinks a lot some weekends. On her birthday, she usually drinks more than a lot. Sometimes, she blacks the hell out and wakes up not remembering that she tried to fight someone on the street and then sobbed a little later at a club when the DJ went old school and played Ma$e's "Feel So Good" because…memories. At least, until the next morning when she wonders what she's done—and how long her body will be paying for it.

The Reality: Chances are, if you've ever done too many shots of tequila while partying, you've woken up the next day wondering, "What happened last night? Why is my body covered in glitter? Where did I get this sweet foam finger and also, this gnarly bruise?"

Blackouts are shockingly common: Research has previously shown that more than half of college students experience alcohol-induced amnesia at least once in their lives. (Forty percent of those surveyed had blacked out at least once in the year before the study was done.)

There are two types of blackouts: "fragmentary" and "en bloc." Fragmentary blackouts—also known as "brownouts"—are more common. That's when you remember the night before in bits and pieces, like a movie montage. They tend to come into focus when a friend recalls your amazing rendition of "Hotline Bling"—at what was not a karaoke bar. En bloc blackouts are the real deal; Don't go looking for memories, because they ain't there.

When you drink too fast—spiking your blood alcohol level in the process—alcohol suppresses functions in the areas of the brain that create memories, says Reagan R. Wetherill, a professor of psychiatry at the University of Pennsylvania who studies the effects of heavy drinking. Brain cells within the hippocampus, which is responsible for memory, stop functioning—which means long-term memories aren't created.

"Alcohol primarily seems to interfere with the transfer of information from short-term to long-term memory," says James H. Fallon, a professor emeritus of anatomy, neurobiology and psychiatry at the University of California School of Medicine. When the hippocampus isn't functioning well, you're only living in the present. "It's almost like we would think of some insects or frogs—they live in the moment, but they don't transfer it into what would be the type of long-term memory responsible for storing personal experiences."

Another problem: You can't predict when you're going to blackout. It's not just the amount you drink and your metabolism, it's also your genes, Fallon says.

The Worst That Could Happen: If she can't remember it, it never happened, right? Um, nah. Blackouts aren't to be taken lightly—especially if they become a pattern. During a blackout, Wetherill explains, "other parts of the brain are still functioning—likely in an impaired way—so you can still respond to your environment, but you're not creating memories for what you're saying or doing." That's why you can appear totally functional when you're talking to your boss at the company Christmas party and not recall having the conversation.

But let's be real: The worst that could happen isn't that she makes a fool of herself in front of the CEO. Losing control is one thing, but she's also leaving herself vulnerable. People who black out often report experiencing dangerous, traumatic incidents—everything from getting into fights to being the victims of sexual assault. And because of the ability to appear perfectly normal, there are also a lot of questions regarding the implications for consent during sex.

What Will Probably Happen: If this is a fairly isolated incident, she'll likely drunk text her ex, make herself a pizza, and cry herself to sleep watching a lame rom-com. Then she'll wake up happy that she didn't burn her apartment down. Either way, she'll probably need to block out a few hours tomorrow to deal with the inevitable hangover. What You Should Tell Your Friend: If you want to help her deal with the guilt, you might remind her that women are more susceptible to blackouts because they're smaller, they don't have as much water in their bodies, or the enzyme in their gut that breaks down alcohol before it's absorbed. Women also tend to drink beverages like fruity mixed drinks, tequila drinks, and even Rosé that have higher concentrations of alcohol. Translation: you're going to get to a fun place a lot faster.

Next time, make sure she knows not to drink on an empty stomach, skips the shots—or the beer chugging contest—and anything else that will cause her BAC to spike. Her odds of blacking out also increase if she mixes alcohol with marijuana or benzos since they, too, inhibit parts of the brain. And while recurring blackouts could be a sign of alcoholism, it's not a sure thing, Wetherill says. Diagnosing alcoholism is more complicated than that—so for the time being, focus on helping her dial it wayyyyy back.