The percentage of Americans who take painkillers stronger than morphine is on the rise, according to a new report by the Centers for Disease Control and Prevention (CDC). These drugs include oxycodone, hydromorphone, fentanyl and methadone.

The study found 37 percent of people who used a prescription narcotic painkiller in 2011-2012 used a drug stronger than morphine, compared with 17 percent in 1999, USA Today reports.

An estimated seven percent of adults use a narcotic painkiller, according to the CDC. Women are more likely than men to use opioid painkillers. The findings indicate that the use of opioid analgesics among U.S. adults has more than doubled since 1988–1994, when 3.4 percent used opioid painkillers, the CDC noted.

Non-Hispanic white adults were more likely than Hispanic adults to use the drugs, the study found. There was no significant difference in use between non-Hispanic white adults and non-Hispanic black adults.

The CDC noted that opioid dependence and opioid-related deaths are a growing public health problems. Opioid painkiller sales quadrupled from 1999 to 2010. From 1999 to 2012, opioid-related deaths more than tripled.