A new study finds fentanyl deaths have increased in states west of the Mississippi River, according to NPR.

The researchers say fentanyl has spread westward, increasing deaths in the short-term and threatening to dramatically worsen the nation’s already severe opioid epidemic in the long-term.

The study found fentanyl deaths have increased in cities including Los Angeles, Phoenix and Seattle. “Up through 2018, the vast majority of synthetic opioid overdoses occurred east of the Mississippi River,” said study author Chelsea Shover, an epidemiologist at Stanford University.

Some people have died after using drugs that they did not know were contaminated with fentanyl. “You think you’re using heroin or you think you’re using Ecstasy or Xanax or what looks like an OxyContin pill, but it’s actually fentanyl,” Shover said.