The drug behind the recent overdoses at Portlaoise Prison has been identified as a new synthetic benzodiazepine.
Eleven prisoners were hospitalised on Tuesday, with three taken to the ICU, after taking the drug, known as clobromazolam.
This is the first time this drug has been identified in Ireland.
The HSE Emerging Drug Trends laboratory confirmed that there were no opioids involved, specifically no nitazenes.
Professor Eamon Keenan, HSE addiction services' clinical lead, said the new synthetic benzodiazepine is extremely potent and is a sedative type of drug.
He said: "The situation at Portlaoise Prison recently is an example of how volatile the drug market is. New substances are appearing on the market and causing harm quickly."
The HSE said clobromazolam has not been identified in other community settings.
