Three people were hospitalised in Sydney after taking tablets they believed to be MDMA but that actually contained a potent opioid.

Health authorities in NSW have issued an urgent warning about the tablets, which are orange or red and rectangular, with a red bull logo and wording.

They were sold as MDMA but testing by NSW Health revealed they instead contained nitazene, a potent opioid.

Three people presented to Sydney hospitals over the weekend after taking as little as half a tablet.

One patient was admitted to the intensive care unit but has since been discharged.

NSW Poisons Information Centre medical director Darren Roberts said drugs containing potent opioids such as nitazenes can cause unexpected and severe overdose or death.

"Nitazenes can be as strong, or stronger, than fentanyl and may be more likely to impair breathing than other opioids," Dr Roberts said.

"It's important people recognise the signs of an opioid overdose early and know how to respond. Taking the appropriate action early can save a life."

Opioid overdose symptoms can include pin-point pupils, drowsiness, loss of consciousness, slow breathing or snoring or skin turning blue or grey.

Anyone who witnesses someone experiencing these symptoms after using drugs should call triple zero immediately.

The ACT introduced a fixed pill-testing site in July 2022 as part of a trial due to run until the end of 2024, while Queensland has also paved the way for fixed and mobile drug-testing sites.

NSW has so far resisted rolling out a similar regime, a measure that is expected to be considered at a state drug summit this year, although no date has been announced for the forum.