Two people are dead and seven others seriously ill amid fears that a batch of very high purity heroin is in circulation in Cork.
Ambulances have been called to nine cases in the past 24 hours which is causing major concern for medics.
Doctors are awaiting toxicology results but have said it is important to issue an urgent warning before anyone else is affected.
Dr Chris Luke from Cork University Hospital said all opiate users need to be on the alert.
Today's news comes after last month's deaths of two young men in Kinsale in Co Cork. Michael Coleman and Liam Coffey were found dead in Michael's rented house in Abbey Lane in Kinsale on Sunday, September 9.
Gardaí confirmed that a product containing MDMA or ecstasy and another substance PMMA may have led to their sudden deaths.
The HSE took the unusual step of warning drug users against taking the substance, which is known to contain Methylamphetamine (MDMA Ecstasy) and Paramethoxymethamphetamine (PMMA).
