Frantic passers-by called 999 when the 17-year-old fell ill in front of a crowd of people in Glasgow city centre.
He first collapsed on Friday evening in Union Street and was rushed to Glasgow Royal Infirmary.
The following night he was back in hospital after taking another suspected legal high.
On Sunday, he once again had to be treated by medics in hospital after swallowing another substance.
Police said all three incidents took place in busy Union Street.
Chief Inspector Alan Porte today warned youngsters are playing a "dangerous game".
Officers were also called to a report of a disturbance near Union Street and found another 17-year-old male unconscious.
It is believed he had also taken a legal high and was taken to Glasgow Royal Infirmary, shortly after 7pm on Friday.
Mr Porte, area commander for Glasgow city centre, said: "Anyone who takes these so-called legal highs is putting their life in danger.
"The risk is increased further when mixed with other drugs and alcohol.
"By taking them, you are putting your life in the hands of others, who are more concerned with making money."
So-called legal highs can cause drowsiness, hallucinations, coma, paranoid states, seizures and even death.
The Evening Times previously revealed how an increasing number of legal highs contain illegal drugs.
Justice Secretary Kenny MacAskill hosted a summit in Glasgow last week on how to disrupt the sale and supply of the substances.
He joined police, trading standards and Home Office staff to discuss how to deal with the increasing problems posed by legal highs.
Mr MacAskill warned: "Just because they are legal doesn't mean they are safe.
"Just because they are not illegal doesn't mean they can't cost you your life."
Substances sold as legal highs are created to have similar effects to existing illegal drugs but they fall outside the UK government's misuse of drugs laws.
Earlier this month, the Evening Times told how a 14-year-old schoolboy was rushed to hospital after taking a legal high.
In 2012, there were 36 deaths in Scotland, where legal highs were found during the toxicology report -three times the death rate from ecstasy.