Overdoses are expected to spike in urban and rural areas because more people are mixing synthetic opioids such as fentanyl with methamphetamines and cocaine, according to a new study.

Researchers at Northwestern Medicine in Chicago analyzed opioid death data from the Centers for Disease Control and Prevention between 1999 and 2020. They found opioid overdose deaths in 2020 rose more quickly in rural areas than in urban areas. Between 2019 and 2020, overdose death rates rose for the first time in six types of urban and rural counties, HealthDay reports.

According to the researchers, opioid overdoses have reached historic highs because people are combining synthetic opioids with stimulants such as cocaine and methamphetamines. This combination is difficult to reverse during an overdose, they said.

“I’m sounding the alarm because, for the first time, there is a convergence and escalation of acceleration rates for every type of rural and urban county,” study author Lori Post said in a news release. “Not only is the death rate from an opioid at an all-time high, but the acceleration of that death rate signals explosive exponential growth that is even larger than an already historic high.”