Nearly one-third of Medicare patients who get prescriptions for powerful narcotic painkillers receive them from multiple doctors, raising the risk of injury and death, according to a new study.

What’s even more concerning regarding the findings is that many of the doctors were unaware that their patients were already being prescribed prescription drugs.

Narcotics, which include painkillers such as hydrocodone, oxycodone and morphine, remain a major issue for communities were abuse remains a concern. Prescriptions for these drugs have risen sharply in the United States in the past 20 years, leading to an increase in the number of reported overdoses.  Researchers have found that being prescribed painkillers by multiple doctors could be a source of the problem.

“As physicians, we tell patients not to drive when they take opioids, but we also need to tell them that it can be dangerous to receive these medications from more than one provider,” said study author Dr. Anupam Jena, an assistant professor of health care policy at Harvard Medical School.

Jena and his colleagues also found that having multiple doctors prescribe prescription painkillers increased a patient’s risk of  hospitalization for drug-related complications, such as breathing problems, drowsiness and injuries from falls.

The new study examined over 1.2 million medical records of Medicare patients who had received a prescription opioid. It showed that nearly 35 percent received a prescription from more than one doctor. It also suggested that close to a third of the group received prescriptions from four or more doctors, according to The Boston Globe.

Many health systems and state governments are creating tools to make it easier for doctors to determine if patients are already getting prescription painkillers from another doctor.

Prescriptions for narcotics in the United States increased nearly three-fold from 1991 to 2009, to more than 200 million a year, according to the National Institutes of Health. Narcotic overuse and abuse is a major health issue in the country.