An analysis of more than a quarter million urine samples found that 60 percent of patients tested misused their prescribed drugs -- either skipping medications, or combining them with other prescriptions or illegal drugs.

The data come from Quest Diagnostics, and the samples were submitted by primary care doctors as well as specialists.

Among patients who had misused medications, marijuana was the most commonly abused drug, followed by prescription pain killers and anti-anxiety medications.

According to the Quest report, 45 percent of patients who used marijuana recreationally also used other non-prescribed drugs, most commonly sedatives and narcotic pain killers, compared to 36% of non-marijuana users.

"This suggests that recreational marijuana users are more likely to use other illict drugs," said Dr. Leland McClure, director of pain management for Quest Diagnostics. He added that the findings contradict the notion that marijuana is a "harmless drug."

Adam Brooks of the Treatment Research Institute in Philadelphia says the Quest Diagnostics findings show how important it is for doctors to discuss drug use with patients. "It points out to me that if you are operating in primary care and you're not having these conversations, you are missing the boat," Brooks said. "Despite the fact that there has been some reluctance to put screening and intervention in primary care, this just speaks to the fact that it's greatly needed."

Brooks says physicians are pressed for time, and don't receive a lot of training on handling drug abuse issues. "This is difficult to talk about, it can be embarrassing to talk about with a patient, so it's easier to push this to the back of the list of things to discuss with a patient."

Brooks says integrating the issue of substance abuse into primary care has been a very slow process.