The alert follows news that 10 teenagers ended up in hospital after taking the mind-bending drug, which causes hallucinations and is similar to LSD and ecstasy.
Three arrests were made after one fell seriously ill. The youths took the drug at two separate parties nine miles apart on Friday.
One was in a critical care unit until yesterday and several others remain under observation.
The teens were partying at Croxley Green and Potten End in Hertfordshire when they became ill and were taken to Watford General Hospital.
Two men aged 18 and 20 as well as a 20-year-old woman were held on suspicion of dealing. One of those arrested - who were all freed on bail - also needed hospital treatment.
It is thought the victims had all overdosed on the drug.
Hertfordshire Police warned users were 'dicing with death' by taking Rocket Fuel.
Detective Inspector Paul Doran said: "If you become unwell having taken this, go to hospital immediately."
More on Parentdish: Talking to your children about drugs and legal highs
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