Nearly 30 percent of fatal opioid overdoses also involve benzodiazepines, such as Xanax, Valium or Klonopin, researchers at Stanford University have found.

“It’s not news that this combination is not a good one, but despite being well known, it’s gone up over time, and more people are ending up in the hospital because of it,” lead researcher Dr. Eric Sun said. “Patients and doctors really need to think twice about this combination.”

The findings come from a study of data from more than 300,000 patients who were prescribed a narcotic painkiller between 2001 and 2013, HealthDay reports. The researchers report in BMJ that 9 percent of those patients also had prescriptions for benzodiazepines in 2001. By 2013, that had risen to 17 percent.

The findings raise the possibility that some of the increase in opioid-related deaths might be caused by increases in concurrent benzodiazepine/opioid use, the researchers wrote.