Smoking a joint once a week or a bit more apparently doesn't harm the lungs.

That's according to new evidence from a 20-year federally funded health study that finds marijuana doesn't do the kind of damage tobacco does.

Marijuana is the most commonly used illegal drug in the U.S. although some states allow it to be used for medical purposes.

Researchers from the University of California, San Francisco and the University of Alabama at Birmingham studied a national database to determine the long-term lung damage caused by smoking marijuana.

Their analysis showed pot did not appear to harm lung function, but cigarettes did.

In fact, they found that occasional pot use actually improves lung air flow rates and lung capacity.

The evidence, however, was not strong enough to recommend marijuana as a treatment for better lung health.

Experts also warn that using marijuana increases the risk for heart attack and can impair the immune system.