CHICAGO (Reuters) - A study of more than 5,000 youngsters in Switzerland has found those who smoked marijuana do as well or better in some areas as those who don't, researchers said on Monday.

But the same was not true for those who used both tobacco and marijuana, who tended to be heavier users of the drug, said the report from Dr. J.C. Suris and colleagues at the University of Lausanne.

The study did not confirm the hypothesis that those who abstained from marijuana and tobacco functioned better overall, the authors said.

In fact, those who used only marijuana were "more socially driven ... significantly more likely to practice sports and they have a better relationship with their peers" than abstainers, it said.

"Moreover, even though they are more likely to skip class, they have the same level of good grades; and although they have a worse relationship with their parents, they are not more likely to be depressed" than abstainers, it added.

It did not explain the reasons behind the apparent effect.

The study, published in the November issue of the Archives of Pediatrics & Adolescent Medicine, was based on a 2002 survey of 5,263 Swiss students age 16 to 20, of whom 455 smoked marijuana only, 1,703 who used both marijuana and tobacco and 3,105 who abstained from both.

The report said that while marijuana use has declined among U.S. adolescents, it has increased in recent years among the same age group in Switzerland and other European countries.

The study said that while one theory holds that using legal drugs like nicotine and alcohol opens the door to marijuana and other illegal drug use, recent research also has found marijuana may come first and it "may reinforce cigarette smoking or lead to nicotine addiction ..."

In the study, about half of the tobacco and marijuana group had used the latter drug 10 times or more in the previous month. That compared to 56 percent in the marijuana-only group who had used the drug only once or twice in the same time period.

"These findings agree with previous research indicating that (tobacco) smokers were significantly more likely to be heavy cannabis users than nonsmokers," the study concluded.

In addition, those who use only marijuana were less likely to have started using that drug before the age of 15 compared to tobacco users, and the tobacco-marijuana group was more likely to have abused alcohol, the study said.

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