A study of heavy marijuana users found that about one-third reported resuming use of the drug to relieve or avoid withdrawal symptoms, according to researcher David Gorelick, M.D., Ph.D., of the National Institute on Drug Abuse.

"Heavy pot users should be aware that they may experience a withdrawal syndrome that will make them uncomfortable when they try to quit," he said.

WebMD reported May 7 that Gorelick said at the annual meeting of the American Psychiatric Association that the study involving about 500 heavy, long-term marijuana users -- about a quarter of whom reported smoking marijuana more than 10,000 times during their lifetime -- found that 42.4 percent of those studied reported at least one symptom of withdrawal, such as cravings, irritability, boredom, anxiety, or sleep disturbances when they abstained from use.

Not all of these users, however, resumed marijuana use as a result.

Gorelick said he expects marijuana-withdrawal syndrome to be included as a psychiatric disorder in the next edition of the Diagnostic and Statistical Manual of Mental Disorders, due in 2012.