Science/Medicine
2:26 pm
Thu January 12, 2012

Study: Smoking marijuana does not impair lung function

As the debate continues in Michigan over how to enforce the state's medical marijuana law, a study published in the Journal of the American Medical Association concludes that regularly smoking marijuana does not impair lung function.

Here is one of the co-authors of the research, Dr. Stefan Kertesz, explaining what they found.

Dr. Kertesz is an associate professor at the University of Alabama at Birmingham School of Medicine and a physician at the Birmingham VA Medical Center.

He says other studies on the subject showed mixed results.

"What this study clarifies," Kertesz explains in the video, "is that the relationship to marijuana and lung function changes depending on how much a person has taken in over the course of a lifetime."

"At those very high levels of use, there could be harm. However, at those lower or moderate levels of use… there’s no real evidence of harm to air flow rate or to lung capacity," said Kertesz.

"Moderate levels of marijuana use" as defined in this study was not all that moderate. It was the equivalent of  smoking up to a joint a day for seven years.

Anahad O'Conner reports in the New York Times blog "Well":

The new research is one of the most extensive looks to date at whether long-term marijuana use causes pulmonary damage, and specifically whether its impact on the lungs is as harmful as smoking cigarettes. The researchers followed more than 5,000 people over two decades and found that regularly smoking marijuana — the equivalent of up to a joint a day over seven years — did not impair performance on a lung function test. The test, a measure of pulmonary obstruction that looks at the amount of air a person can force out in one second after taking a deep breath, is typically worsened by smoking tobacco.

The results are surprising as marijuana smoke contains similar noxious ingredients as tobacco smoke, which is widely known to impair lung function.

The researchers "suspect some kind of training effect from repeated deep inhalations" of marijuana smoke.

In the Well blog, O'Conner writes that Dr. Donald Tashkin, a pulminologist from the University of California, has another possible explanation:

[Tashkin] said one reason marijuana smoke may not be as harmful as tobacco smoke, despite containing similar noxious ingredients, may be the fact that its active ingredient, THC, has anti-inflammatory effects.

“We don’t know for sure,” he said, “but a very reasonable possibility is that THC may actually interfere with the development of chronic obstructive pulmonary disease.”

Dr. Kertesz points out that this study only looked at lung function. He said marijuana is a complicated substance.

"It can affect someone's tendency get into risky situations. It can affect someone's likelihood of getting into an accidents. It can affect fertility," said Kertesz. He also said that at "high levels of use," it's still possible that it could cause harm to lung function.