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Smoking marijuana once a week will not damage a person's lungs, a 20-year study has found.

Researchers at the University of California, San Francisco (UCSF), and the University of Alabama at Birmingham found that marijuana cigarettes appeared to have less impact on lung function than ordinary cigarettes, despite containing some of the same toxic chemicals as tobacco.

Unexpectedly, the rate of air flow through the lungs was found to increase rather than decrease with marijuana use up to a certain level, according to a report published in the Journal of the American Medical Association.

The study, one of the biggest and longest of its type, will bolster arguments for the legalisation of marijuana, which is already acceptable in some states for medical purposes.

More than 5,000 men and women between the ages of 18 and 30 took part in the federally funded study over a 20-year period from 1985-2005. The data has only just been analysed.

The participants received regular lung function tests. About 37 percent of them admitted that they smoked marijuana in small amounts - one or two joints a month - along with cigarettes.

Analysis found that while cigarettes decreased lung capacity as expected, marijuana smokers reported enhanced lung capacity. One researcher speculated that that might be due to the practice of holding one's breath after inhaling cannabis smoke to maximise its effects.

Marijuana smokers who indulged once a day over the course of seven years also did not experience a decline in lung function, though the increased air flow associated with casual use levelled off with heavier smokers.

"We found exactly what we thought we would find in relation to tobacco exposure - a consistent loss of function with increasing exposure," said lead author Mark Pletcher, associate professor in the division of clinical epidemiology at UCSF.

"We were, however, surprised that we found such a different pattern of association with marijuana exposure."

Despite the results, the researchers advised caution and moderation when considering marijuana use.