What Causes Blackouts?

People faint when there’s too little blood flow to the brain.

When a person says he’s blacked out, he could mean one of several things. He might have lost consciousness for a short time. If so, he could also say he passed out or fainted. Another possibility is he might have stayed awake, continued talking, moving, or even driving but then later not remember what he’d done.

People who’ve had the second kind of blackout may remember bits and pieces of the blackout period. Or, they might remember with reminders. If that’s the case, the event is known as a partial blackout or brownout. If the person can’t remember anything, that’s a complete or total blackout.

Here is an explanation of what could cause blackouts and information to help you know what you need to do if you or someone you’re with has one.

The most common cause of a complete or partial blackout is alcohol. When you drink while taking other drugs, a blackout is even more likely. Frequent alcohol-related blackouts might be a sign of problem drinking.

If you or someone you know has blackouts that involve loss of memory or consciousness and you don’t know why, see a doctor right away. Blackouts can be a sign of a serious medical condition.

If you usually stop after a drink or two, then you’ve probably never had an alcohol-related blackout. They usually happen when people drink too much too fast. Consuming excessive alcohol can lead to gaps in memory. Those gaps form because the alcohol prevents short-term memories from becoming long-term memories.

Sometimes, people who drink too much may have spotty memories of what happened the night before. Other times, someone who’s heavily intoxicated may lose hours of memory. If the memories never formed in the first place, there’s no way to get them back.

Who’s at Risk for Alcohol-Related Blackouts?

People who binge drink -- consuming four to five drinks in about two hours -- are more likely to have alcohol-related blackouts. When people drink this much, blood alcohol levels may reach or exceed 0.08 or higher, which is the legal limit to be considered impaired. The more quickly alcohol enters the bloodstream, the more likely a person is to black out.

Alcohol-related blackouts can happen to anyone, but certain things like the following can make them more likely:

• Taking prescription medications or other drugs while drinking
• Drinking on an empty stomach
• Drinking too fast
• Being younger aged, which makes a person more likely to binge drink and to overestimate how much he can handle
• Lower weight

Alcohol-related blackouts generally don’t cause more widespread loss of memory. But blackouts can be very dangerous. That’s because people who’ve blacked out don’t know what they are doing and may do things they otherwise wouldn’t.

The consequences may be embarrassing, or they could be dangerous. For example, you could become a victim of a violent crime. Blackouts may even be life-threatening. If you drive a car, for instance, you not only put your own life at risk but the lives of those around you. The same is true for becoming involved in illegal or risky activities.

You don’t have to have a drinking problem to black out. But, blackouts may indicate an alcohol use disorder. If you or someone you know blacks out often while drinking, get help.

Sometimes, seizures cause a loss of consciousness described as a blackout.

More often, people faint when there’s too little blood flow to the brain. Several things can reduce blood flow and cause you to pass out, including:

• Certain prescription medications
• Low blood pressure
• Giving blood
• Standing too long, especially in a warm or crowded room
• Dehydration
• Intense fear or stress
• Diabetes
• Heart problems, including abnormal heart rhythm

Frequent alcohol-related blackouts can be a sign of a serious drinking problem. But the good news is treatment can help.

If you have blackouts for unknown reasons, see your doctor for tests. A doctor will want to rule out heart problems that could be life-threatening. But, for many people who faint, there’s not a dangerous health problem to blame. Sometimes, doctors will not find any cause for the blackout. Some of those people will faint again within the next three years. Others, though, will never faint again.