In fact, occasional marijuana use was associated with slight increases in lung airflow rates and increases in lung volume, the study found.
Far from a license to light up, the study eases the worry among some health professionals that daily use of marijuana for medical reasons could have negative, long-term implications on pulmonary health.
The study, led by Mark Pletcher of the University of California, San Francisco, compared the effects of both cigarette and marijuana smoking over a period of 20 years in a group of more than 5,000 adults, part of a longitudinal study called Coronary Artery Risk Development in Young Adults (CARDIA).
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.
Not so with marijuana. More 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 at levels of marijuana smoking higher than this did the researchers see a leveling off and then potential reversal of this relationship to improved lung capacity.
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 milliliters, or the size of a kid's juice box.
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 told LiveScience. The airflow increase, then, 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.
"A marijuana smoker might have a few joints a month, or a small number of joints or pipe bowls a day," Kertesz said. "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."
But before you start lighting up, do remember you have other body parts. The main active chemical in marijuana is delta-9-tetrahydrocannabinol, or THC. THC travels to the brain, producing that high. The long-term effects of THC 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. As reported earlier this month in the journal Archives of General Psychiatry, THC may trigger psychosis by interfering with the brain's ability to distinguish between important and unimportant stimuli that constantly flood the various regions in the brain.
Marijuana can hinder learning and memory, too, for weeks after exposure. This implies that casual weekend smokers might always be functioning at a suboptimal level.
A study released Jan. 9 in the journal Neurogastroenterology and Motility even found that marijuana use is linked to a disorder called cyclic vomiting, which may be related to migraines and causes people to experience hours- or days-long vomiting episodes.
"Marijuana is complicated," Kertesz said. "It could be affecting your social life, your work life or even your tendency to get into accidents."
Christopher Wanjek is the author of the books "Bad Medicine" and "Food At Work." His column, Bad Medicine, appears regularly on LiveScience.