NSW Health put out a warning on November 26, suggesting that the drug - commonly known as 'purp' - was potentially being distributed in both metropolitan and regional parts of the state.

Anyone who may come into contact with the drug, or who suspects they have seen it, is warned not to touch it under any circumstances. NSW Health stated that accidental exposure to powdered fentanyl can have serious health consequences.

NSW Chief Addiction Medicine Specialist, Dr Tony Gill, said a number of people in NSW who recently used heroin developed toxicity from acetylfentanyl and fentanyl. Some described the heroin as being purple in colour.

"We've seen a number of people recently where fentanyl was taken unknowingly and was associated with serious harm. Separately to the heroin related cases, another cluster of fentanyl and acetylfentanyl has been associated with cocaine use, similar to those seen in October of this year," Dr Gill said.

"Fentanyl can cause drowsiness, loss of consciousness and slowed breathing, and when taken unknowingly can be life-threatening."

Fentanyl is a strong opioid that is used for a range of health conditions, primarily for the management of severe pain. Acetylfentanyl is a similar opioid to fentanyl and has similar effects but is not used medically.

"It's important that people realise an overdose can occur with very small doses of fentanyl-related substances. The severity will depend on the amount of fentanyl or acetylfentanyl within a particular substance and how much people take," Dr Gill said.