A warning has been issued after a 15-year-old boy was hospitalised after he was said to have taken mephedrone.
The drug, which has been illegal since 2010, is also known as MCAT and meow meow.
Users can feel euphoric, very alert and talkative, but some will feel anxious and paranoid.
Gwent Police say the boy from Cwmbran has now been discharged but added the drug is very addictive and its long-term side effects are largely unknown.
Insp Nick McLain, of Torfaen local policing unit, said: "There are some serious side effects to using meow meow.
"These can include heart attack, seizures, sweating and paranoia.
"Apart from the health dangers, being caught in possession of meow meow could mean up to a five-year jail sentence."
BBC NEWS 13th September 2012