Narcotic painkillers can be highly-dangerous in certain combinations, and a new study shows most American patients taking the medicines are consuming potent mixtures of the drugs.

Express Scripts researchers examined records of 6.8 million Americans, finding that in 60 percent of cases, patients were taking potentially hazardous combinations of medicines. Nearly two-thirds of the subjects taking these mixtures were females.  

Around two-thirds of patients consuming hazardous mixtures of drugs received prescriptions from more than one physicians, according to the study. Of those people taking two or more medicines, almost 40 percent had their orders filled by more than one pharmacy.

The study is published in a new report, covering the years 2009 to 2013.

A Nation in Pain examined narcotic painkillers, including morphine, oxycodone, hydrocodone, and codeine. Elderly people were found to have the highest rate of opiate use, while younger patients refilled prescriptions more frequently.  

Nearly one-third of patients taking mixtures of drugs consumed an anti-anxiety drug in the benzodiazepine family. This pairing is the most common mixture in fatalities caused from drug combinations.

More than one quarter - 28 percent - of those people taking hazardous drug mixtures were consuming muscle relaxants along with the opiate.

Around 27 percent of the mixtures involved combinations of various opioids. A potent combination of opiate, benzodiazipene, and muscle relaxant - known as a Houston cocktail - was consumed by eight percent of the patients.

"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, medical director at Express Scripts, said.

The quantity of drugs taken by patients in the United States went up over the last five years, while the number of patients taking narcotic painkillers are becoming more numerous in the United States over the last five years.

Researchers discovered almost half of the patients studied the consumed short-acting drugs over a long period. This behavior greatly increases the risk of addiction to the narcotic. Almost half of patients who took opioid painkillers in the first year after initial prescription were still taking the drugs three years later, according to the report.

Pain medication usage was found to be most common in small cities in the southeastern United States, the study found.

"America claims less than 5% of the world's population, yet it consumes roughly 80% of the world's opioid supply. Knowing the potential for misuse of these medications, and facing an increase in opioid-related deaths in this country, we wanted a deeper understanding of how patients in the U.S. are using these medications," A Nation in Pain reported.