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Heavy weed smoking during adolescence does NOT increase boys' risk of developing mental or physical health problems later in life, astonishing new research has claimed.

The findings fly in the face of previous studies which claimed cannabis use can alter teens' brain structure and ruin their memory.

Researchers from the University of Pittsburgh Medical Center and Rutgers University tracked 408 males from adolescence into their mid-30s.

They expected to see teenage cannabis users face an increased risk of developing lung cancer, asthma, depression or psychotic symptoms including delusions and hallucinations.

But they found no connection between illness and weed smoking whatsoever.

(Image: Getty)

The study is one of only a handful which has traced the lives of cannabis smokers for more than two decades.

"What we found was a little surprising," said lead researcher Jordan Bechtold, a psychology research fellow at the University of Pittsburgh Medical Center.

"There were no differences in any of the mental or physical health outcomes that we measured regardless of the amount or frequency of marijuana used during adolescence."

(Image: PA)

One "striking" finding of the research suggested that even the men who were "chronic" smokers - lighting up several times a week - faced no increased risk of mental or physical illnesses.

Previous studies have found long term weed smokers are more likely to become depressed than their sober friends.

But in his study, Bechtold claimed this link may have been "inflated".

Cannabis is now being legalised in many states across America and there are many vocal pro-pot voices across the world.

"We wanted to help inform the debate about legalization of marijuana, but it's a very complicated issue and one study should not be taken in isolation," Bechtold said.