The HSE has issued a warning to drug users following five drug-related deaths in Cork and Dublin in recent weeks.
In a statement this evening the HSE says it seems the deaths occurred after users either smoked or injected the non-pharmaceutical drug fentanyls.
Fentanyls can be up to 600 times more potent than morphine and may be sold as "designer" fentanyl or "synthetic" heroin.
The HSE has warned that even the smallest amount of the substance can cause overdose and death and that even experienced heroin users are at risk if they take this drug.
To date five deaths are being investigated in Ireland where Fentanyls has been implicated.