A sharp increase in the use of anti-anxiety drugs such as Valium and Xanax is raising concerns among Twin Cities doctors and national researchers, who say it's contributing to overdose deaths that are masked by the larger epidemic of painkiller abuse.

The number of Americans filling prescriptions for benzodiazepines, often referred to as "benzos," rose 67 percent between 1996 and 2013, according to a new study. Overdoses involving the drugs rose fourfold during that period.

That trend may have been overlooked because hospitals were seeing so many opioid-related overdoses, said Dr. Marcus Bachhuber, the study's author and an assistant professor at Montefiore Medical Center and Albert Einstein College of Medicine in New York.

"We didn't totally understand that many people weren't taking them by themselves," Bachhuber said. "They were taking them with opioids or alcohol or other medications."

In 2014, 55 Minnesotans died as a result of overdoses involving benzodiazepines, up from 12 in 2000, according to federal data reviewed by Bachhuber. Last year, Minnesotans filled more than 1.6 million prescriptions for the drugs, according to the state Board of Pharmacy.

Although Minnesota's overdose numbers lag behind the national rates, the growing number of deaths has raised alarms for health professionals here.

Joseph Lee, a medical director at the Hazelden Betty Ford Foundation, said he's noticed a troubling spike in benzodiazepine abuse and points to doctors with lax prescribing practices.

"There are clinicians that are being careless about prescribing benzodiazepines," Lee said. Noting that it hasn't received the same scrutiny as opioid use, he added: "Now the data is saying we might want to take a closer look at it."

One cause may be busy doctors facing pressure to satisfy patients after brief visits, said Dr. Charles Reznikoff, an addiction specialist at Hennepin County Medical Center.

"Doctors need the patient to feel that they've done something for them, but I only have 10 minutes," Reznikoff said. "If you put a doc under that kind of pressure, what are they supposed to do?"

Coordinating prescriptions also is a challenge for patients who have one doctor prescribing painkillers and another prescribing other medications, Reznikoff said. "It's kind of like the left hand doesn't know what the right hand is doing," he said.

Reznikoff speculates that drug tolerance could be a factor among patients who begin using other drugs after the euphoric effects of benzos wear off. For people with past addiction problems, prescribing benzos can be tricky because of effects that some patients find pleasurable.

"It's like alcohol in a pill," Lee said. Some opioid users, he added, take benzos to either enhance their high or ease the discomfort when coming down from painkillers or heroin.

Withdrawal from benzos can be dangerous and often requires medical supervision because, like alcohol withdrawal, it can cause fatal seizures and other health problems. "When you take people off of benzodiazepines, they get angry," Lee said.

Tracking deaths caused by benzodiazepines is difficult because patients often use them with painkillers and their death is listed as opioid related, Reznikoff said. And because benzos can make people drowsy, they may be contributing to traffic fatalities that are not tracked as drug-related, he said.

Lori Hedican, an investigator in the Ramsey County medical examiner's office, said she also has noticed an increase in overdoses caused by opioids and benzodiazepines combined. But if her office finds a fatal amount of heroin in a corpse, for example, examiners will rule it an opioid death.

Overall, tracking the use and abuse of anxiety drugs is difficult because privacy-minded legislators have imposed limits on the amount of data the state can keep, said Cody Wiberg, executive director of the state Pharmacy Board.

"There needs to be constant vigilance for people that are willing to take drugs that will kill them," Wiberg said. "I don't know if you can stop people from doing it."

Youssef Rddad is a University of Minnesota student on assignment for the Star Tribune.