The HSE has warned heroin users to be "extra cautious" following more than 20 overdoses in Dublin in 24 hours.
The health service said it is monitoring the cases and awaiting "analytical confirmation on the situation", while asking people to avoid buying from new sources and new batches of heroin "as there is increased risk at this moment in time".
Professor Eamon Keenan, HSE National Clinical Lead, Addiction Services, said: "We are aware of a number of overdoses occurring in Dublin city today relating to potential new sources of heroin.
"Dublin Fire Brigade Ambulance Service have informed us of over 20 overdose situations that they have responded to in 24 hours."
While not every call out would result in a person being taken to hospital, Prof Keenan said that the emergency department in St James's Hospital saw "over twice the number" of overdose presentations "than would normally be seen".
"The other thing that we have noticed is that overdoses have required a higher among of Naloxone, which is the antidote," Prof Keenan said.
Prof Keenan added: "We ask that people are extra cautious during this period while we identify what may be leading to overdoses.
"We ask service providers to notify their service users who may be at increased risk and help support them to reduce harms during this time.
"We will update services and people who use drugs following analytical confirmation of samples obtained which will help clarify the situation.
"At present, this issue appears contained to the Dublin city area but we will continue to monitor and update as necessary."
The HSE has asked drugs users to follow harm reduction steps, including accessing overdose medication naloxone and not using heroin alone.
It also recommends testing the dose by starting "low and slow" and avoiding other drugs, including methadone, benzodiazepines or alcohol.
Look out for signs of an overdose and get immediate medical help if someone is unwell, the HSE said.
We are now in a public health crisis - Doolan
Dr Austin O'Carroll confirmed that a Dublin service that he works with, which provides GP care and addiction services to homeless people, treated two people who suffered from heroin overdoses this morning.
He said: "At ten o'clock my manager found someone had overdosed, and he had Naloxone in his pocket and administered it immediately.
"We managed to revive that person with a second Naloxone. We had a second person overdose within a few minutes.
"The ambulance came and they told us that they had seen seven overdoses already and that was by ten o'clock this morning."
Dr O'Carroll said that one of the patients that had overdosed was taken to hospital and the other chose not to go.
Dr O'Carroll said the service contacted the HSE and Dublin Regional Homeless Executive to inform of the overdoses "and they immediately sprung into action to let people know of the risks".
Obtained samples of the drug are currently being analysed.
"The fear is that there could be a synthetic opiod, not fentanyl, but there are synthetic opiods in the UK called nitazenes which may be the possibility in this and that's what we want to outrule," Dr O'Carroll said.
Eddie Mullins, Chief Executive of Merchant's Quay Ireland, which provides addiction services, said the charity has received an alert from the HSE.
However Mr Mullins confirmed to RTÉ News that they have not "as yet" seen any increase in overdoses among those using its services in Dublin's south inner city.
Sinn Féin councillor Daithí Doolan said it was "a public health crisis".
Speaking to RTÉ News, Mr Doolan said there was a need for supervised injection sites.
Be aware. There is a bad batch of heroin in Dublin. There have been 13 overdoses today. Please let people know. If you're going to use dont use on your own. If someone overdoses diall 999. pic.twitter.com/D18PtnENzC
— Cllr. Daithí Doolan (@DaithiDoolan) November 9, 2023
"It reinforces the need to have drug testing, so we can actually test the drugs and then supervise the consumption rooms as a matter of urgency," he said.
"This is a public health issue, this is not a criminal issue. We are now in a public health crisis.
"We need to make sure there is a public health response to these people as well."
Contact the HSE Drug and Alcohol Helpline on 1800 459 459.
