People who occasionally smoke marijuana do not suffer long-term lung damage the way cigarette smokers do, and may actually experience a slight improvement, said a 20-year US study published Tuesday.
Since the research included more than 5,000 people over a long time span, the authors said it should help clear up some of the confusion about the risks of marijuana smoking, which is increasingly common in the United States.
However, they warned that the risks of heavy marijuana use were difficult to assess and cautioned against regular or frequent smoking.
"Marijuana is still an illegal drug, and it has many complicated effects on the human body and its function," said Stefan Kertesz, senior author of the research in the Journal of the American Medical Association.
"In our findings we see hints of harm in pulmonary function with heavy use, and other studies have shown that marijuana use increases a user's likelihood of a heart attack... and impairs the immune system's ability to fight disease."
The data came from the Coronary Artery Risk Development in Young Adults Study (CARDIA), funded by the National Heart, Lung and Blood Institute, which aimed to examine heart disease and how it develops.
Researchers recruited subjects between the ages of 18-30 in four US cities and followed them from 1985 to 2006.
They measured marijuana use with a methodology called "joint-years," in which one joint-year of exposure would be the equivalent of smoking 365 joints or pipe bowls.
For those who reported smoking an average of one joint a day for seven years, or one joint/week for 49 years, the study found no harmful lung effects resulted.
"With up to seven joint-years of life-time exposure, we found no evidence that increasing exposure to marijuana adversely affects pulmonary function," the study said.
Researchers at the University of Alabama Division of Preventive Medicine and at the Veterans Affairs Medical Center in Birmingham measured lung function by assessing the force and volume of each person's exhale.
Those who smoked cigarettes tended to have much higher lifetime exposure to smoking than marijuana users, but when researchers adjusted the data for current and lifetime exposure, they found smokers' lung function got worse over time while pot smokers' lung function actually improved slightly.
"At levels of marijuana exposure commonly seen in Americans, occasional marijuana use was associated with increases in lung air flow rates and increases in lung capacity," said Kertesz.
However, the gains -- while statistically significant -- were so small that most people would likely not notice at all, he added.
"It's not enough of an increase that would make you feel better," he said.
"Healthy adults can blow out three to four liters of air in one second. The amount of gain, on average, from marijuana is small, 50 ccs or roughly a fifth of a can of coke. So it's not something that would be noticeable."
According to government figures, marijuana is the most commonly used illicit drug in the United States. Some 16.7 million Americans ages 12 and older reported using marijuana at least once in the month prior to being surveyed in 2009.
Medical use of marijuana is now legal in 16 states and the US capital region to alleviate the symptoms of cancer, AIDS and glaucoma.