Police say a powerful fentanyl-based drug, which is 100 times more powerful than street heroin, is being sold in Newport.
Officers have seen increased reports of sales and use of the drug, known as 'China White'.
Gwent Police says investigations have revealed sales are taking place in the city centre and the Pill area.
'China White' is a class-A synthetic drug that is light brown in colour - it is upwards of 100 times more potent than street heroin and officers have found it being sold in its place with users being unaware.
In February, drug dealer Kyle Enos, 25, was jailed for eight years for selling fentanyl to customers – including four in the UK who have since died. Enos used the dark web to import, supply and export fentanyl.
Officers who searched his home and arrested him wore bio-hazard suits and respirators due to the danger of the drug.
The court heard that following his arrest police went through Enos’ customer database due to the “potentially lethal effects” of fentanyl.
Four people on his database had died, though it cannot be proven that the fentanyl supplied by Enos was related to their deaths.
On his online advert Enos stated: “I sell seriously strong, potent drugs.”
Fentanyl is a synthetic opioid sometimes prescribed as a painkiller for the terminally ill. The National Crime Agency (NCA) revealed last year the drug was linked to 60 UK deaths over eight months.
At that time, Ifor Glyn, regional director of charity Drugaid Cymru, said: “For the second consecutive year, Wales saw an increase in the number of individuals dying as a result of a drugs overdose, and my fear is we could be looking at the beginning of an upward trend as they are currently seeing in north America with the drug fentanyl.
“The amount of deaths they are seeing is unprecedented and a major worry. It was only this week we heard of an individual found guilty in South Wales of supplying fentanyl. This drug could be the greatest threat we have seen yet, and could have devastating consequences.”
Police say drug dealers putting fentanyl in heroin can be handed more substantial sentences in court as guidelines make it clear that adding toxic cutting agents to drugs is deemed an 'aggravating' feature.