Almost 60 percent of patients who take narcotic painkillers to treat long-term conditions are also being prescribed other drugs, such as anti-anxiety medication or muscle relaxants, which could cause dangerous interactions, a new study finds.

Combining an opioid painkiller with these drugs could slow down the respiratory system. About 8 percent of patients were taking painkillers, anti-anxiety drugs and muscle relaxants at the same time. The study found 27 percent of patients were taking more than one opioid at a time, which is also hazardous, the article notes.

Two-thirds of the patients taking a combination of drugs were being prescribed the medications by two or more doctors. Almost 40 percent filled their prescriptions at more than one pharmacy. Patients who are taking narcotic painkillers are using more of them for longer periods, the study found. Almost half of patients who took the painkillers for more than a month were still using the drugs three years later. This is a sign of potential abuse, according to The New York Times.

The study was conducted by the pharmacy benefits manager Express Scripts. The researchers examined pharmacy claims of 6.8 million Americans who filled at least one prescription for an opioid painkiller between 2009 and 2013.

“There could be instances when prescribing these combinations of drugs is appropriate, but not at this scale. The fact that the majority of these patients are being treated by multiple physicians and pharmacies signals a communication breakdown that leads to dangerous use,” Lynne Nowak, MD, Medical Director at Express Scripts, said in a news release. “Government- and insurer-run drug monitoring programs can help prevent these possibly life-threatening scenarios, but unfortunately they are underused and vary by state. As more people gain access to health coverage, this problem will worsen if the country doesn’t use every tool at its disposal to ensure the safe use of these medications.”