Emergency rooms in many states reported a decrease in heroin overdose cases between 2017 and 2018, HealthDay reports.

Researchers from the Centers for Disease Control and Prevention looked at emergency department data from 22 states and Washington, D.C., and found an overall decrease of 21.5 percent in heroin overdose visits.

Some states that have been hardest hit by the opioid epidemic, including West Virginia, Ohio and Washington, D.C., experienced decreases of more than 50 percent, the researchers wrote in the American Journal of Public Health.

Kentucky, Maryland, Massachusetts, New Hampshire, Pennsylvania, Rhode Island and Wisconsin also had significant decreases in hospital-treated heroin overdoses.

Three states—Illinois, Indiana and Utah—had significant increases in heroin overdoses treated in the ER. Lead researcher Alana Vivolo-Kantor said these increases may be related to growing availability of fentanyl in the heroin supply in those states.