The 13-year-old boy collapsed in Cardiff after smoking King Cobra, South Wales Police said.
It comes after officers carried out a drugs raid in Morriston, Swansea, last week and seized Â£6,000 worth of new psychoactive substances (NPS).
Police said Swansea council's trading standards team is now investigating.
Det Ch Insp Jason Redrup said: "Across south Wales we are seeing an increase in people using NPS, especially among young people.
"It is important that people know that these substances are not only illegal but are also dangerous to their health. In most cases users won't know exactly what they are taking."
Referring to the drugs warrant executed at a commercial premises at Woodfield Street in Morriston, police neighbourhood beat manager PC Richard Williams said: "We have received a constant flow of information from the local community about this premises which is linked to recent publicity about the use of so-called legal highs.
"We have acted on this information and by working with our partners in trading standards have disrupted this operation."
Legal highs which are not under any form of controls are sometimes labelled as "research chemicals", "herbal incense" or "not for human consumption".
Nearly 350 new psychoactive substances have been identified, the UN's Office on Drugs and Crime (UNODC) said earlier this month.

Play was called off by umpires Nigel Cowley and Billy Taylor at 18:00 BST.
A day-and-a-half has now been lost, play having been abandoned on Tuesday, when the rain arrived just in time to prevent Durham starting their second innings 170 runs adrift.
If play resumes on Thursday, Durham and Worcestershire will both have replacements playing.
Durham's Keaton Jennings and Worcestershire's Joe Clarke are now on England Lions duty, meaning call-ups for Michael Richardson and Tom Fell respectively.
Durham will delay a decision about their choice of captain for the forthcoming Twenty20 Blast until they discover whether Jennings, an England Test candidate, will be available to them.

The child, who cannot be named for legal reasons, was taken into foster care in Gloucestershire while on a supervised visit with its father.
The parents should have been given 14 days notice, and "serious errors" were made by the local authority, a family court ruled.
Gloucestershire County Council has apologised and said it had now updated its policies.
Judge Stephen Wildblood, in the family court in Gloucester, found the authority's actions were unlawful and had breached the family's human rights.
He said the authority did not pay sufficient regard to a care plan and a court's care order, it failed to comply with regulations, and no legal advice was sought before the child was taken into care last month.
The mother, who is in her early 20s, told the BBC it had been "incredibly traumatic and distressing" and that she hoped no other family had to suffer in the way that she and her child did
In a statement the council said it apologised "unreservedly".
"In our attempts to safeguard the welfare of this child, members of our children's social care team breached the terms of a previous court order.
"We also apologise to the child and the parents for our actions in this case and for any distress caused.
"This should never have happened and we have updated our policies and put in place safeguards, including ensuring that the assistant director or other senior officer will have oversight of all cases where action to remove children, who are subject to care orders, are being considered."
The child, who is aged under two years old, has since been returned to its mother.
A warning over the dangers of taking so-called legal highs has been issued by police after a teenager was taken to hospital.
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No play was possible on day three of Durham's County Championship Division Two meeting with Worcestershire.
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A baby was removed from its vulnerable mother unlawfully, a judge has ruled.