Marijuana linked to brain changes, but are they harmful?

Marijuana may not be physically addictive like nicotine, but many experts agree that it can become a habit that's very hard to break, especially after long-term use. Addiction, as defined by the National Institute on Drug Abuse, is compulsive use even when the user says it's getting in the way of home, work or school.

(CBS) Can pot rot your brain? A preliminary study reveals brain abnormalities in everyday pot smokers, and its author has a bit of advice for marijuana mavens:

Ditch the doobies.

"With this study, we were able to show for the first time that people who abuse cannabis have abnormalities of the cannabinoid receptors," study author Dr. Jussi Hirvonen, adjunct professor of experimental nuclear medicine at Finland's Turku University, said in a written statement.

What in the heck are cannabinoid receptors?

They're the sites on the brain which different substances bind to in order to affect pleasure, concentration, perception of time, memory, sensory perception, and coordination. Marijuana's main psychoactive component  - tetrahydrocannibinol, or THC - is just one of several substances that bind to cannabinoid receptors.

The findings were presented at a medical conference in San Antonio, Tuesday.

For the study, scientists scanned the brains of 30 daily pot smokers to nonsmokers. They found that daily tokers' cannabinoid receptor levels were almost 20 percent lower than those of nonsmokers. 

Fourteen of the stoners quit for a month, and follow-up scans showed that their cannabinoid receptor had returned to near-normal levels. Hivonen called that "good news." But the question remains - just how bad is it to have reduced cannabinoid receptor activity in the first place? Is it really a form of brain damage?

"You have to call it change, not damage," Dr. Marc Galanter, director of the division of alcoholism and drug at New York University Langone Medical Center in New York City, told CBS News. "If you give someone an antidepressant, it changes their neurotransmitters - it's not necessarily damage."

There's no evidence that cannabinoid receptor activity affects behavior, Dr. Galanter said. But he called the study significant, because the findings help scientists better understand the mechanism of effect of THC on the brain.

Marijuana is the nation's most abused illicit drug. More than 16.7 million Americans over age 12 use it, according to the 2009 National Survey on Drug Use and Health. Previous studies have linked marijuana use to an increased risk for heart attacks, and lung and digestive tract cancers.

What do you think?  Does smoking pot damage the brain, or is Dr. Hirvonen blowing smoke?