Seven overdoses that all occurred within hours of one another in North Seattle on Thursday could be a sign of contaminated drugs, health officials say.

Public Health Seattle King County is currently working to figure out what happened.

“It takes a while to get the final results of the drug testing to see whether these drug were contaminated with fentanyl or otherwise more potent than the users anticipated, which appears to be the case,” Public Health’s Dr. Jeff Duchin said.

Six of the seven people who overdosed were transported to the hospital for care. One person had reportedly injected heroin; another reported snorting crushed pills.

Overdoses like this are on the rise, but Duchin said that people can take steps to protect themselves. One protective measure is to carry naloxone, the medication used to reverse the effects of an overdose.

Another protection: Never use drugs alone.