Five women overdosed within a 24-hour period at Eagle River's Hiland Mountain Correctional Center earlier this week, the Alaska Department of Corrections said.
One woman overdosed Monday, and the remaining four overdosed Tuesday, spokeswoman Megan Edge said.
Nurses and corrections officers at the prison "began performing lifesaving measures while they waited for the arrival of emergency medical responders," Edge wrote in a release.
In four of the incidents, medical responders gave the women Narcan, which counters the effect of opioid overdoses, Edge wrote.
Edge said the agency was still trying to determine exactly what drug caused the overdoses.
"We know that Narcan was able to revive and help stabilize them. We're still trying to determine exactly what drug was used," Edge said.
All of the women were revived and responsive by the time they were taken to a hospital, according to Edge.
"Alaska is in the midst of a terrible opioid epidemic, and our prisons are not immune to that," state corrections commissioner Dean Williams said in the release.
An internal investigation was underway, Edge said.
"Drug overdoses aren't unheard of in prisons, but to have 5 back-to-back, that is very uncommon," Edge said.