Warning over spate of 'severe opioid overdoses'

NSW Health said nitazenes had been found in drug samples related to the cluster in the past month around Penrith.

"Nitazenes are potent synthetic opioids which can be stronger than fentanyl and hundreds of times more potent than heroin," the public health warning said.

"They are more likely to decrease or stop breathing than other opioids."

Investigations into the cluster of cases in the Nepean Blue Mountains Local Health District are ongoing.

NSW Health chief addiction medicine specialist Dr Hester Wilson said it was important people recognise the signs of opioid overdose early.

"Opioids such as heroin can cause pin-point pupils, drowsiness, loss of consciousness, slowed breathing/snoring and skin turning blue/grey and can be life-threatening," Dr Wilson said.

"One of the dangers of illicit drug supply is the strength and contents of the substance you are getting is unknown and can be inconsistent.

"In light of this detection, people who use drugs such as heroin should carry naloxone."

Naxalone can temporarily reverse an opioid overdose and is considered a life-saving measure.

It comes in the form of a nasal spray or injection from some health services.

Anyone who uses it is advised to call Triple Zero for an ambulance immediately for follow-up care.

"If you feel unwell, or if your friend feels unwell, do something about it," the public health warning said.