A few hits on the bong now and then don't seem to have any detrimental effects on lung health, suggests a new study.

Researchers found that multiple measures of lung function actually improved slightly as young people reported using more marijuana - at least up to a couple thousand lifetime joints.

"There's no doubt, if you've watched a Harold & Kumar movie, marijuana triggers a cough," said Dr Stefan Kertesz, from the University of Alabama at Birmingham, who worked on the new study. But questions have remained about the drug's longer-term effect on lung functioning.

"Previous studies have had mixed results," Kertesz explained. "Some have hinted at an increase in lung air flow rates and lung volume (with marijuana smoking), and others have not found that. Others have found hints of harm."

While marijuana smoke has a lot of the same toxins as cigarette smoke, he added, people who use pot tend to smoke fewer joints each day than tobacco users smoke cigarettes. That and the method of inhaling may offer some relative lung protection, researchers have proposed.

Still, the findings don't let marijuana off the hook for long-term health consequences.

"I think a lot more work will need to be done to make any blanket statements about safety," said Dr Jeanette Tetrault, a substance abuse researcher at the Yale School of Medicine in New Haven, who wasn't tied to the new research.

"These are only two measures of pulmonary function and don't really paint the entire picture" of the potential effects of marijuana on the lungs, she told Reuters Health.

The new data come from a long-term study of more than 5000 young adults in Oakland, Chicago, Minneapolis and Birmingham. From 1985 until 2006, researchers regularly asked participants about their past and current use of cigarettes and marijuana. They also tested how much air their lungs could hold and the maximum rate of air flow out of their lungs.

Not surprisingly, the researchers found that the more cigarettes their participants smoked or had smoked in the past, the worse their lungs performed on both tests, Kertesz's team reported in the Journal of the American Medical Association.

But at least at moderate levels of pot smoking, that didn't seem to be the case - in fact, the trend was reversed.

Lung volume and air flow rates both increased with each "joint-year" - the equivalent of 365 joints or pipe bowls - participants said they'd ever smoked, up until about seven joint-years, or some 2555 joints.

It was a small overall improvement, though. Lung airflow - measured by how much air people could blow out in one second - was no more than 50 milliliters higher in pot smokers compared with non-smokers. The average value for a healthy male is four litres, according to Kertesz.

"It's a very real increase ... but it's so small that I don't think that a person would feel a benefit in terms of their breathing," Kertesz said.

"Is this a real increase in lung health? That's the other question. We don't know exactly what's happening inside all of the airways of the individuals who are measured."

One explanation, he said, could be that deep breaths taken by pot smokers when they're inhaling could train them to do well on a test of lung function that involves breathing in and blowing out air as quickly as possible.

That wouldn't necessarily mean their lungs are better off for marathon running, for example.

At the highest levels of pot smoking - using marijuana more than 20 times in a month, or having over 10 lifelong joint-years worth of smoking - lung function seemed to decline again, but the researchers noted that there weren't enough heavy marijuana users in their study population to be sure of that.

Of course, researchers said, the new findings don't mean people should reach for the bong to boost their lung capacity.

Pot might irritate the lungs in the short term and cause problems for people with asthma, they said. And there's more to consider besides breathing - like traffic accidents and problems on the job or at school while under the influence.

Whether pot smoking may increase the risks of certain types of cancer is still controversial.

But it's unlikely that it puts users at risk of chronic obstructive pulmonary disease, or COPD, as smoking does, according to Dr Donald Tashkin, who studies the effects of marijuana on the lungs at the University of California, Los Angeles but wasn't involved in the new study.

When it comes to diminished lung function, "This particular potential complication of marijuana smoking doesn't appear to be an important risk," he told Reuters Health.

"Therefore, people who are using marijuana for medicinal purposes or recreationally at least could be reassured that they're not harming their lungs in this way."