Fatal overdoses involving common anxiety drugs like Xanax and Valium are at an all-time high, according to new research that has many mental health experts scratching their heads and wondering why.

In a new study, researchers found that prescriptions for benzodiazepines used to treat anxiety, mood disorders, and insomnia in more than 5 percent of Americans tripled from 1996 to 2013, while overdoses quadrupled, reports Newser.

In 2013, "benzos" played a role in nearly one in three of the nation's 22,000 prescription overdose deaths. The rise in OD deaths suggests that "people have been taking them in a riskier way over time," said lead author Marcus Bachhuber.

Experts speculate that the spike in deaths is linked to higher doses of the drugs, or that people are taking them carelessly along with alcohol or other drugs, increasing the risk they pose.

The death rate leveled off among whites between 2010 and 2013, but rose among blacks and Hispanics, the researchers found.

The study’s authors caution that benzodiazepines are being over-prescribed in general and suggest that doctors look into alternatives drugs or treatments such as talk therapy.