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

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.

“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…

Read the rest of the story from our partners at NBC News

Get our Health Newsletter. Sign up to receive the latest health and science news, plus answers to wellness questions and expert tips. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Contact us at letters@time.com.