Teen Internet addiction can predict past or present drug abuse. (Credit: Thinkstock/Getty Images)

Parents already panicky about the amount of time their teenage children spend online may now have something new to worry about: All those hours spent Web surfing, chatting, gaming, texting and posting to Facebook could be a warning sign of substance abuse, according to a new study in the March issue of the Journal of Addiction Medicine.

Greek researchers found that teenagers with "pathologic" Internet use were more likely to admit to drug abuse, and as excessive Internet use  increased, so did the likelihood of substance abuse. The study also linked substance abuse and excessive Internet use to such personality traits as nonconformity, aggressiveness, recklessness and impulsiveness.

"Not only did we find that specific personality attributes were important in both substance abuse and Internet addiction, but that Internet addiction remained an important predictor of substance abuse," study co-author Georgios Floros, at Aristotle University of Thessaloniki, said in an email to ABCNews.com.

Floros and colleagues surveyed 1,271 students between the ages of 14 and 19 on the Aegean island of Kos about their Internet use, substance use and personality. To determine who was "Internet addicted," the researchers administered  a 20-question "Internet addiction test" that asked how often the students  stayed online longer than they'd intended, how often their grades or studies slipped because of the amount of line spent online, how often they'd "yell, snap or act annoyed" if someone bothered them while they were online.

When they compared the mean values of "illicit substance abuse" among the teenage participants, the researchers  found that those who reported substance abuse  had "significantly" higher mean scores on the Internet addiction test, and that those scores were important predictors for substance use, either past or present.

"The predictive element showed an interesting new finding," said Floros.

"It's not a shocking result to me," David Greenfield, a Connecticut psychologist and founder of the Center for Internet and Technology Addiction, told ABCNews.com. "The study offers another set of variables to look at when doing a workup."

Dr. Megan Moreno, a pediatric and adolescent medicine specialist at UW Health in Madison, Wis., said, "I've definitely seen kids who showed signs of problematic Internet use. Some of them do go on to have other problem behaviors. Sometimes that's substance abuse, sometimes it's other addictive behaviors, like excess exercise or excess shopping."

Parents might wonder when they should start to worry about their "Internet addict" kids.  At what point does  mere gadget fixation morph into something  more "pathologic" or "addictive"? And what is "pathologic" Internet use?

There's no agreed-upon answer. Internet addiction is not a recognized formal diagnosis, and holds no place in the Diagnostic and Statistical Manual of Mental Disorders - the psychiatrists' "bible" - although Internet addiction could make it into the appendix of the manual's new edition as a special disorder that requires more research, according to the American Psychiatric Association.

For now, medical practitioners turn to the symptoms of other addictive disorders, such as gambling and substance abuse - compulsiveness, lack of control, failed attempts to cut back, disregard for obvious negative consequences - to diagnose Internet addiction.

Not that they're all the same.  "When somebody develops an addiction toward something, our usual recommendation is, 'don't do it,'" Moreno told ABCNews.com. "We tell the alcoholic not to drink. We tell the gambler not to go to a casino. But what can you do if someone has a problem with Internet addiction. What kind of job could they get? How are they going to function?"

Contrary to what parents might believe, the amount of time their teen  spends, say, playing "World of Warcraft" or getting lost in the virtual fantasy of "Second Life" is not what's key.

"It's a factor, but it's not the only factor," said Moreno. "A lot has to do with that kid's relationship to the Internet. Do they feel the day is horrible if they can't get online? If they're offline, are they constantly thinking about going online? Are they substituting things that they can do offline and only doing them online? I worry about the kids who only make friends online."

And before parents freak too much over this latest heady headline notching up the danger in the Internet  zone, they should keep in mind that while this latest study found links between drug use, Internet use and personality type - that's all they are.

"Correlation data is not causative," said Greenfield. "You can't assume because the person has one marker that they're going to have the final issue. But you do need to be aware of what your child is doing online and how much they're doing it."