Anxiety drug overdoses have quadrupled over last two decades, study finds

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More Americans than ever are overdosing on anxiety drugs, researchers reported Thursday — and it's not clear why.

SEE ALSO: A Third of Americans Aren't Getting Enough Sleep

The new study finds not only that more Americans are taking the drugs, which include brand names such as Valium and Xanax, but that they're taking more of them.

But while the quantity of prescriptions filled tripled between 1996 and 2013, the number of overdoses quadrupled during the same period, the team reported in the American Journal of Public Health.

See images from an overdose murder case:

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Lisa Tseng convicted in landmark overdose case
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Anxiety drug overdoses have quadrupled over last two decades, study finds
Dr Hsiu Ying "Lisa" Tseng cries during her arraignment, flanked by attornies, Edward Welbourn, left, and Alan Stokke, Friday, March 16, 2012 in Los Angeles. Tseng, a California doctor, has pleaded not guilty to charges of second-degree murder in the prescription drug overdose deaths of three patients. (AP Photo/Nick Ut)
LOS ANGELES, CA. - OCTOBER 14: The prosecution shows young men who died from prescription drug overdoses written by Dr.Hsiu-Ying 'Lisa' Tseng during the prosecution's closing arguments in the trial of Hsiu-Ying 'Lisa' Tseng in Los Angeles County Superior Court on October 14, 2015 in Los Angeles, California. The Rowland Heights doctor is charged in the deaths of a string of perception medicine overdoses of men in their 20's. (Photo by Anne Cusack/Los Angeles Times via Getty Images)
FILE - In this March 16, 2012 file photo Dr. Lisa Tseng listens in court during her arraignment in Los Angeles. Attorneys are set to deliver opening statements Monday, Aug. 31, in the trial of Tseng, charged with murder the deaths of three young men who overdosed on prescription pain killers. Tseng has pleaded not guilty to three counts of second-degree murder. She could face up to life in prison if convicted on all the charges against her. (AP Photo/Nick Ut, File)
April Rovero, the mother of victim Joey Rovero, speaks to reporters after Dr. Hsiu Ying "Lisa" Tseng was convicted of second-degree murder in his death and the killings of two other patients who died of drug overdoses from pills she prescribed, in Los Angeles Friday, Oct. 30, 2015. The Rowland Heights doctor, not seen, was convicted Friday of three counts of second-degree murder for prescribing massive quantities of addictive and dangerous drugs to patients with no legitimate need, three of whom died of overdoses, the Los Angeles County District Attorney's Office announced. (AP Photo/Brian Melley)
Hsiu Ying 'Lisa'' Tseng, a Rowland Heights doctor charged with secondÂdegree murder and other counts in the prescription drug overdose deaths of three male patients in their 20s, appears in a Los Angeles courtroom with lawyer Donald B. Marks (left) for a preliminary hearing on Monday, June 4, 2012. (Photo by Luis Sinco/Los Angeles Times via Getty Images)
Dr. Hsiu-Ying "Lisa" Tseng appears in court Friday March 9, 2012, in Los Angeles. Tseng is charged with three counts of second-degree murder in the prescription drug overdose deaths of patients. She faces 21 other felony counts. (AP Photo/Nick Ut)
Kelle and Bruce Stavron, huddled together near a photo of their son Matthew who died of an overdose of drugs like those he got from Dr.Lisa Tseng. Photo of Mathew was shot just 3 days before he passed away. He was happy, sober, very much in love and engaged to be married at the time of photo taken. (Photo by Irfan Khan/Los Angeles Times via Getty Images)
Doctor Hsiu Ying "Lisa" Tseng is shown in court, Friday March 2, 2012, to face murder charges, as her arraignment was postponed until March 9, in Los Angeles. The prosecutor took the rare step of charging this doctor, Tseng, with murder in the prescription drug overdose deaths of three patients. (AP Photo/Nick Ut)
Hsiu Ying 'Lisa'' Tseng, a Rowland Heights doctor charged with secondÂdegree murder and other counts in the prescription drug overdose deaths of three male patients in their 20s, appears in a Los Angeles courtroom for a preliminary hearing on Monday, June 4, 2012. (Photo by Luis Sinco/Los Angeles Times via Getty Images)
FILE - In this Friday, March 16, 2012, file photo, Dr Hsiu Ying "Lisa" Tseng listens in court during her arraignment, in Los Angeles. The Rowland Heights doctor was convicted Friday, Oct. 30, 2015, of three counts of second-degree murder for prescribing massive quantities of addictive and dangerous drugs to patients with no legitimate need, three of whom died of overdoses, the Los Angeles County District Attorney's Office announced. (AP Photo/Nick Ut, File)
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"We found that the death rate from overdoses involving benzodiazepines, also known as 'benzos,' has increased more than four-fold since 1996 — a public health problem that has gone under the radar," said Dr. Marcus Bachhuber of the Albert Einstein College of Medicine in New York, who helped lead the study.

"Overdoses from benzodiazepines have increased at a much faster rate than prescriptions for the drugs, indicating that people have been taking them in a riskier way over time."

Benzodiazepines are extremely popular drugs in the U.S.

Related: Drug Overdose Deaths Hit Alarming High

More than 5 percent of U.S. adults fill a benzodiazepine prescription every year, for conditions such as anxiety, mood disorders and insomnia.

They're known to be highly addictive and, in 2013, nearly a third of the 23,000 people who died from prescription drug overdoses were taking them.

Bachhuber's team looked at large health surveys to find trends in their use and abuse.

"The rate of overdose deaths involving benzodiazepines increased more than four-fold from 0.58 per 100,000 adults to 3.07 per 100 000 adults," they wrote. "However, this rate appeared to plateau after 2010."

"Between 1996 and 2013, the number of adults filling a benzodiazepine prescription increased 67 percent, from 8.1 million to 13.5 million," they added.

They found a similarly large increase in the number of pills each adult was prescribed.

It's not clear why overdoses went up so much. It could be people are taking the drugs for longer times, raising the odds that they'll eventually overdose. Or it could be the pills are getting to people who don't have prescriptions, the researchers wrote.

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