Proponents argue that it eases pain and could be useful in the treatment of multiple sclerosis, cancer and HIV. Others say it’s an illegal drug with the potential for long-term damage. But a new Yale study on marijuana may bring scientists one step closer to the answer — especially after a breakthrough court ruling that could lift a key limitation on future research.

Yale medical school researchers weighed in on the debate over marijuana’s effect on the respiratory system, synthesizing the findings of 34 articles in a variety of medical journals from 1966 to 2005. Their conclusions, which question the link between long-term marijuana use and pulmonary disease, were published in the Archive of Internal Medicine on Monday, the same day that a judge recommended lifting a 65-year ban on privately growing marijuana for research purposes.

The researchers found that long-term exposure to marijuana smoke may result in many of the same health problems caused by tobacco smoke, such as coughing, wheezing and phlegm. But, they said, there is no direct evidence linking long-term marijuana use to other smoking-related diseases like emphysema and lung cancer. But eleven of the 12 relevant studies examined found that marijuana smoke leads to the relaxation and opening of air passages in the short term, an argument medical marijuana proponents look to for support.

Still, the authors said, more research is necessary before doctors will know enough about cannabis to prescribe it like other drugs.

“There are no other drugs on the market that a doctor would say, ‘smoke two of these and tell me how you feel in the morning,’” said Brent Moore, a psychiatry professor at the School of Medicine who worked on the study.

Medicinal marijuana has become a hot-button issue in recent years, and scientists looking to contribute research on the subject have begun to speak out about the availability, or lack thereof, of cannabis for use in research. For the past 65 years, the National Institute on Drug Abuse has monopolized the sale of cannabis to all interested researchers. This may soon change, however, as a Drug Enforcement Agency judge recommended that a University of Massachusetts professor be granted a license to grow research-grade marijuana for use in privately funded studies.

Most of Yale’s research on marijuana has been limited to using subjects currently dealing with substance abuse problems, Moore said. The subjects, usually found through newspaper advertisements and other traditional recruitment avenues, must have previously used drugs.

But research into the long-term effects of marijuana has a number of limitations, the report said. For one, it is difficult to account for the possibility that participants have a history of tobacco smoking. In addition, there are no standardized measures for dose and duration of use, said Jeanette Tetrault, Yale researcher and first author of the study. All of the study’s authors agreed that more research into the effects of marijuana must be conducted before any conclusions can be drawn.

“Maybe smoking marijuana is the problem,” Tetrault said. “Maybe the compound itself has positive effects. Maybe we need to focus on delivery. More research still needs to be done.”

Many scientists said they saw the judge’s ruling as a breakthrough in the process of marijuana research, increasing the availability of the drug for use in privately funded studies. The DEA has 20 days either to accept or reject the court’s recommendation to allow the UMass professor to start growing cannabis.