Police have issued a warning after several youths fell ill after taking the drug mephedrone.
The drug is commonly known as "bubble", "drone" or "legal high", imported mainly from China and sold as a plant fertilizer online, police said.
Five youths on Teesdale have been admitted to hospital in recent weeks after taking mephedrone.
Durham Police said anyone found in possession of mephedrone would be arrested.
Darlington Drug and Alcohol Action Team warned its use can result in severe nose bleeds, nose burns, hallucinations, blood circulation problems, rashes, anxiety and paranoia, fits and delusions.
It can become addictive and if taken with alcohol or cut with other drugs it could result in death, the action team said.
Insp Kevin Tuck said: "An 18-year-old has already died in Sweden, where this drug is now banned. No drug is safe and we believe mephedrone, especially when mixed with alcohol or taken with other substances, could be deadly.
"We know it is not on the radar for most parents and, sadly, because of its use in our area we would expect it to be readily available throughout every town and city in the North East and the UK."
The drug is illegal in Israel, Norway, Finland and Sweden. Psychiatrists in the UK have lobbied for it to be banned here.