Youngsters are being warned they could be "playing Russian roulette" with their lives after a legal high being sold in shops was found to contain deadly chemicals.
Vertex, available in "head shops" and on the internet and advertised as a type of incense, has already left a number of people in hospital in the UK and has been linked to deaths in Europe.
Symptoms include profuse sweating, racing heartbeat, extreme muscle tension, delirious ranting and a very high body temperature.
Police in Manchester issued an urgent health warning today over Vertex, as thousands of people descend on the city for a giant music festival this weekend.
Chief Inspector Gary Simpson, of Greater Manchester Police (GMP), said: "I would urge people not to buy or take this, or any other, of these so-called legal highs and to consider the danger they pose - you could be playing Russian roulette with your life."
Authorities were alerted to the danger after a number of people were admitted to hospital after taking the drug in Cheshire in the past few days.
Lab tests by Cheshire Police on samples of the substance showed it contains a potentially lethal chemical called AB-CHMINACA, which is believed to be responsible for a number of fatalities in Europe.
The force also said it is "extremely concerned" that the source appears to be shops and dealers bulk- purchasing from the Bolton area of Greater Manchester.
Mr Simpson said: "The use of 'legal highs' is a worrying trend and we have been working closely with Cheshire Police to issue urgent warnings after a number of people were hospitalised in Cheshire after taking Vertex.
"This weekend thousands of people will descend upon Manchester for the Parklife Weekender festival and I want to remind people about the potentially lethal dangers of 'legal highs'.
"They are a group of drugs that are not yet illegal, but just because they are legal it doesn't mean they are safe.
"I would advise anyone supplying, using or considering using Vertex to stop immediately before it's too late. If you have taken it, seek medical assistance now.
"If you are at Parklife this weekend and start to feel unwell after taking a 'legal high', please go to medical facility on-site immediately."
Police said their intelligence suggests Vertex is being routinely sold by both shops and street dealers.
Officers are now contacting and visiting a number of shops which supply the drug to advise them to remove it from their shelves.
Anyone who has taken any of these substances is urged to seek medical help immediately.
Assistant Chief Constable Mark Roberts, of Cheshire Police, said: "This has become an increasingly worrying trend that has seen us respond to a number of incidents in the county, particularly over recent days."
Parklife event director Jon Drape said: "Parklife has a zero-tolerance policy to both illegal drugs and 'legal highs'.
"Anyone found with either on entry will be denied entry to the festival. Anyone caught taking, using or selling illegal drugs or legal highs within the festival will be ejected and handed over to Greater Manchester Police."
Police advise that if any person is showing signs of the symptoms to dial 999, put the casualty in the recovery position and ask for immediate medical assistance.