Increased availability of prescription opioids and other medications such as benzodiazepines are linked to an increase in U.S. suicide rates, a new study concludes.

Researchers at the University of Colorado-Boulder analyzed almost 600,000 suicide deaths in the United States between 1990 and 2017, HealthDay reports. Women’s suicide rates by poisoning had been declining during the late 1980s and early 1990s. The researchers found that by 1997 – one year after approval of the long-acting opioid OxyContin – women’s suicide rates by poisoning began to rise by about 2% annually.

“In the late 1990s, the method women often consider using to attempt to end their life suddenly became much more potent and much more available, with devastating consequences,” study author Daniel Simon said in a university news release. The study found that states without prescription drug monitoring programs experienced larger increases in women’s suicide rates from poisonings.

After controlling for other factors, the authors conclude that increased availability of opioids and other prescription drugs like benzodiazepines was solely responsible for driving women’s self-poisoning suicides rates up from 1997 to 2006.