WOODLAND HILLS (CBSLA) — The manager of an extended-stay hotel said the hotel had been deemed safe and that operations had returned back to normal Tuesday night after a massive morning response to what authorities called an example of the ongoing opioid epidemic.

“It’s scary being somewhere where you don’t know what you’re neighbors are doing, for sure,” Mary O’Neill, a hotel guest, said.

But guests at the Woodland Hills hotel found out exactly what some of their neighbors were doing when firefighters arrived Tuesday morning. According to the Los Angeles Fire Department, four people staying at the hotel admitted they had ingested the opioid fentanyl.

“One person was in grave condition, three others showing signs of narcotic overdose,” Battalion Chief Andrew Wordin, of the LAFD, said.

That admission triggered a hazmat response. Fire crews said they believed the threat was contained to the one room, but they brought in special equipment for tests just to make sure the hotel was safe for guests.

“Fentanyl is a very, very potent opiate, and it can cause, in very low doses, euphoria,” Dr. David Levine, of the Keck School of Medicine, said. “But in more high doses or moderate doses, it can cause someone to become comatose and stop breathing and ultimately lead to cardiac arrest.”

Levine is the chief of the toxicology division at USC’s Keck School of Medicine.

“Fentanyl has become relatively cheap, and people are using that to get high,” he said. “And people are also using fentanyl as a cutting agent to cut other drugs, meaning they will dilute the quantity of the other drug by substituting fentanyl.”

Back in October, Orange County authorities had their biggest fentanyl bust ever, 20 pounds of the synthetic opioid, which they said was enough to kill the county’s 3.2 million residents three times over. Fentanyl is so deadly that just a tine amount can kill — even if it’s not ingested.

Many first responders, including firefighters and police officers, carry a medication to reverse the effects of an overdose.