BEIJING, Jan. 11 (Xinhuanet) -- Casual marijuana smoking can cause no long-term damage to lungs, according to a new study in Tuesday's Journal of the American Medical Association.

After a study of 5,115 American adults over two decades, researchers from the University of California, San Francisco found the lung capacity of people who smoked marijuana was not diminished, even among those who smoked once or twice a week.

One of the main reasons of why marijuana smoking is not as harmful as tobacco may be that people generally don't have marijuana as much as they have tobacco, the researchers explained.

But it's also possible, the researchers said, marijuana smoking simply isn't as damaging as tobacco.

This finding should reassure doctors and patients for use marijuana in medical treatment, said Dr. Mark Pletcher, the lead author of the study.

However, that does not mean people from now on can bagin smoking marijuana without worrying anything, the researchers warned.

Marijuana contains a lot of the same toxins as tobacco, and smoker can develop throat irritation, coughs and other disorders for frequent use, they added.