Three schoolboys were taken to hospital in a critical condition after taking so-called legal highs, police said.
The teenagers, all aged 15, required treatment after taking the substances in Queen Elizabeth Gardens in Salisbury, Wiltshire.
They were rushed to Salisbury District Hospital shortly before 5am on March 4 in a critical condition.
One of the boys was considered to be in a life-threatening condition. All three are now recovering.
Inspector Dave Minty, of Wiltshire Police, said: "These drugs may be so-called 'legal highs' but there is no way of knowing what chemicals are in these substances and what the consequences of taking them will be.
"We are working closely with the boys' parents and school, and I would strongly encourage parents/guardians to speak to their children about the risks of taking any drugs, legal or otherwise.
"My officers have commenced an investigation into this incident.
"I would also ask that anyone with information as to where these substances are being sold contacts us straight away."
Anyone with information should call the force on 101 or Crimestoppers anonymously on 0800 555 111.