The report, published yesterday in the Journal of the American Medical Association, found that lung capacity of those who smoked marijuana wasn’t decreased by regular smoking.

Only heavy tokers, who smoked 20 joints or more a month, were affected.

Doctors have always assumed that marijuana smoke carried the same health consequences in the long-term as cigarettes.

But while marijuana has many of the same toxins as cigarette smoke, the study found that it doesn’t have the same risks for lung disease.

Reasons could be that marijuana smokers use the drug less regularly than cigarette smokers smoke and that THC, the chemical in pot that causes a high, has anti-inflammatory properties.

Researchers found that occasional users noticed a small increase in lung capacity, but not one that would be noticeable to the individual.

But Dr Stephen Sidney, a study author with Kaiser Northern California’s division of research in Oakland, said: “This study shouldn’t be interpreted as marijuana is totally harmless.”

Amanda Reiman, a UC Berkeley lecturer and director of research at the Berkeley Patients Group, a medical marijuana dispensary, said: “No one would ever claim that drinking water has the same effect as drinking vodka, even though they’re both liquids and you’re ingesting them the same way.

“But for some reason we have assumed that because we know the negative outcomes with cigarettes, inhaling any plant material is going to have the same outcomes.

“This study is challenging the preconceived notions we’ve had for some time about the dangers of smoking cannabis and the similarities to smoking tobacco,”

But the researchers agree that pot may irritate the lungs in the short term.

Dr Stefan Kertesz from the University of Alabama at Birmingham said: “There’s no doubt, if you’ve watched a Harold & Kumar movie, marijuana triggers a cough.”