More than half of opioid overdose deaths are caused by synthetic drugs, according to the Centers for Disease Control and Prevention (CDC). The agency said the opioid epidemic increasingly involves illicitly manufactured fentanyl and an array of potent compounds that are chemically related to fentanyl, such as carfentanil.

In a warning released this week, the CDC said synthetic opioids accounted for more than 27,000 overdose deaths in the United States in the 12 months ending in November 2017, NBC News reports. There were 20,000 overdose deaths from synthetic opioids in 2016.

“The dramatic rise in the supply of illicitly manufactured fentanyl and fentanyl analogs has been mirrored by an equally dramatic rise in deaths involving synthetic opioids other than methadone, a category which includes fentanyl and fentanyl analogs,” the CDC said in its alert.

People who overdose on synthetic opioids may need multiple doses of naloxone, the CDC noted.