How many teens have "Internet addiction?"

One in every 25 teens reported an "irresistible urge" to be on the Internet, tension when they weren't online, or said they had tried to quit or cut down on Internet time, according to a U.S. study.

In addition, the study of more than 3,500 high school students in the state of Connecticut found that those students with "problematic Internet use" were more likely than their peers to be depressed and aggressive, and to use drugs.

But study leader Timothy Liu, at Yale University, and his colleagues said they couldn't prove a "cause and effect" link between the Internet habits, depression and drug use.

"Problematic Internet use may be present in about 4 percent of high school students in the United States," they wrote in The Journal of Clinical Psychology.

"It may be associated with depression, substance use, and aggressive behaviors. High school boys, though, may have heavier Internet use and may be less self-aware of the related problems."

The study surveyed students at ten different high schools in Connecticut, asking more than 150 questions about health, risky behaviors, and impulsiveness - including seven questions on Internet use.

Teens were asked to say if they had ever missed school or important social activities because they were surfing the Web, or if their family had expressed concern about their time online.

Specifically, three questions were used to determine if a student had "problematic Internet use." They asked students if they ever had an "irresistible urge" to be online, if they had experienced "a growing tension or anxiety that can be relieved only by using the Internet," or if they had tried to quit or cut down.

Out of 3,560 students, 4 percent met the criteria for problematic Internet use. Asian and Hispanic students were more likely to qualify, although the majority of students in the study were white.

Girls were more likely to answer yes to one of the questions, but more boys said they spent in excess of 20 hours a week online - about 17 percent of boys, compared to 13 percent of girls.

Students who were problematic Internet users were according to the survey also tended to be more depressed and would get into serious fights more often. And boys in that category had higher rates of smoking and drug use.

However, they didn't do any worse in school based on their grades, and experts said it was hard to explain the link.

"It often becomes a chicken and egg issue: are they online because they're depressed, or are they depressed because they're spending inordinate amounts of time online?" said Elias Aboujaoude, a Stanford University researcher who was not involved in the study, told Reuters.

He added that preliminary evidence suggests that problematic Internet use shares common features of drug and alcohol abuse disorders, obsessive-compulsive disorders and impulse-control disorders.