'Occasional pot smoking not as harmful as cigarettes'

Occasionally smoking marijuana does not appear to have long-term adverse effects on lung function the way puffing of tobacco does, a new 20-year-long study has claimed.

The study by researchers from the University of Alabama at Birmingham and University of California at San Francisco also found that slight increases in lung airflow rates and in lung volume among those who smoke marijuana less frequently.

For the study, published in the Journal of the American Medical Association, the researchers analysed marijuana and tobacco use among 5,000 adults from the a national database intended to determine heart disease risk factors over the two-decade period.

The data revealed nothing new about tobacco: As exposure to tobacco goes up, lung capacity in terms of the amount of air a smoker can exhale goes down. It's a linear relationship.

But that was not the case with marijuana. Pot smoking was associated with increases in lung capacity up to a level equivalent to about one joint per day for seven years or one joint per week for up to 49 years.

Only the heaviest pot smokers -- more than 20 joints per month -- showed decreased lung function throughout the study, LiveScience reported.

The increase in lung function at low levels was very small, said study researcher Stefan Kertesz, a professor of preventive medicine at the University of Alabama, Birmingham. It was about 50 millilitres, or the size of a kid's juice box.

A marijuana smoker might have a few joints a month, or a small number of joints or pipe bowls a day. That's never going to be quite as much smoke as a tobacco smoker with a half-a-pack, pack-a-day or two packs-a-day habit," Kertesz said.

The researchers don't know why light-to-moderate pot use might subtly improve lung function. It could be that marijuana users inadvertently train themselves to be good at the inhalation and exhalation test because they "practice" deep breathing when they smoke pot, Kertesz said.

The airflow increase, however, is not necessarily an indicator of healthier lungs. At higher levels, this tiny increase seems to disappear and lung function may decrease, Kertesz said.

The study included few heavy users, which may be one key to why pot smoking isn't as harmful as tobacco smoking.

But before you start lighting up, do remember you have other body parts, the researchers warned.

The key chemical in pot is delta-9-tetrahydrocannabinol, or THC, which travels to the brain, producing that high. Its long-term effects are uncertain, but most health professionals don't consider THC to be benign.

Chronic marijuana use has been associated with anxiety, schizophrenia, bipolar disorders and depression. It can also hinder learning and memory for weeks after exposure.

This implies that casual weekend smokers might always be functioning at a suboptimal level, the researchers said.

"Marijuana is complicated. It could be affecting your social life, your work life or even your tendency to get into accidents," Kertesz added.