Among teens who drink, 90 percent have blacked out after drinking at least once by the time they reached age 19, according to a new study of British adolescents. Teens who black out after drinking are more likely to be female.

When a person blacks out, they appear to be awake, alert and intoxicated, but they have no memory of what has happened. At high enough doses, alcohol impairs the acquisition of memory.

Females are more likely to black out because they weigh less and have less body water to dilute the alcohol, the researchers noted. The study included 1,402 teens ages 15 to 19 who drank. Other risk factors for blacking out after drinking included smoking, having sensation-seeking and impulsive behaviors, lacking conscientiousness and having friends who also drank or used other substances, Time reports.

The findings appear in Alcoholism: Clinical & Experimental Research.

“It’s not as if a blackout in these kids was an isolated phenomenon,” said lead researcher Marc Schuckit, professor of psychiatry at the University of California San Diego. “Blackouts are unfortunately often considered to be a funny thing as opposed to dangerous. I am not sure the average person realizes the dangers associated with blackouts.”

In a press release, he noted, “Someone who has had a blackout cannot remember part of their drinking episode. As you can imagine, blackouts are likely to occur when the drinker is vulnerable to a range of additional dangerous consequences. Women might have unprotected sex, place themselves in a situation where they can be raped, or not be fully capable of protecting themselves. Men can get into fights, use very bad judgment regarding another person, and are often the driver when BACs (blood alcohol concentrations) associated with blackouts can lead to a car accident. Blackouts are very dangerous for both men and women.”