Unlike the cigarette habit, occasional pot smoking does not seem to trigger declines in lung function that could lead to breathing problems, a new 20-year study suggests.
"Tobacco takes you down that road toward breathlessness, but low to moderate levels of marijuana don't," said study co-author Dr Stefan Kertesz.
But there were limits to the study. For example, the findings do not indicate whether occasional (two or three joints a month) pot smokers face a higher risk of lung diseases such as cancer.
Nevertheless, the research should fuel the ongoing debate over medical marijuana, which critics say is too hazardous to serve as a drug to treat conditions such as pain.
Effects on people who are not addicted
It is clear that marijuana smoke irritates the lungs, causing coughing and sputum production, and addiction to marijuana obviously causes problems, noted Kertesz, an associate professor at the University of Alabama at Birmingham. However, as with alcohol and other drugs, the question of harm becomes more ambiguous when it comes to more occasional users who aren't addicted, he said.
"What about the adults who use three joints of marijuana a month for many years?" he said. "Clarifying that is actually quite difficult."
In the new study, researchers examined the findings of a study, which began in 1985, of more than 5 100 people aged 18-30 from Oakland, Calif.; Chicago; Minneapolis; and Birmingham, Ala. Every few years over two decades, the participants took lung tests that measured their lung function through their ability to blow hard into a tube.
The typical tobacco user smoked eight to nine cigarettes a day, while the marijuana users in the study smoked about two to three times over the past 30 days. "That's really different from ('pothead' film icons) Cheech and Chong," he said. "Americans who smoke marijuana typically don't smoke it every day."
Respiratory risks did seem to rise with the intensity of marijuana use, however. The study authors found increasing evidence of lung trouble among people who smoked marijuana more heavily (20 or more times a month).
The study findings appear in an issue of the Journal of the American Medical Association.
Kertesz said that research has consistently shown that "with tobacco smokers, the more use of tobacco that's accumulated, the more airflow and lung volume is lost over time." But that wasn't the case with mild to moderate users of marijuana, he and his colleagues found.
Dr Robert Hancox, a New Zealand scientist who has studied marijuana, praised the research. But he also believes that pot smoking has negative respiratory effects.
"Smoking marijuana definitely harms the lungs," said Hancox, an associate professor at the University of Otago's department of preventive and social medicine. "Several studies have clearly shown that even light marijuana smokers can develop severe bronchitis with symptoms of cough and phlegm production. Studies have also demonstrated that smoking marijuana leads to abnormal lung function, but using different tests to those used by this study. What this study shows is that the pattern of lung damage seen with marijuana is not the same as caused by tobacco."
And one advocate against drug abuse noted that the study has a variety of limitations.
"A significant problem is that cannabis use is often difficult to quantify precisely due to smokers sharing joints, different inhalation techniques and different ways of smoking cannabis, including joints, pipes and bongs," said Calvina Fay, executive director of Drug Free America Foundation. "By comparison, the average amount of tobacco in a commercial cigarette of standard length is 1 gram."
Also, Fay said, "it is important to not forget the numerous other serious consequences of marijuana use," such as cognitive and learning problems, psychosis, addiction, criminal behaviour and impaired driving, "none of which were considered in this study."
On the other side of the argument is Paul Armentano, deputy director of the national pro-marijuana group NORML. He said that people can now virtually eliminate the lung-irritating effects of marijuana by using vaporizer devices.
Armentano added that, in his opinion, science has determined alcohol, tobacco and prescription drugs "to be far more dangerous and costlier to society than cannabis," but they are regulated instead of banned.
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