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 taken 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 a substance.
Police said all three incidents took place in busy Union Street.
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 hospital shortly after 7pm on Friday.
Chief Inspector Alan Porte, area commander for Glasgow city centre, said: "Anyone who takes these so-called legal highs is putting their life in danger."
So-called legal highs can cause drowsiness, hallucinations, coma, paranoid states, seizures and even death.
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 problem.
In 2012, there were 36 deaths in Scotland, where legal highs were found during the toxicology report.