The three men, aged 28, 31 and 36, and two women, aged 33 and 40, are said to be in a stable condition after the incident in Heather Avenue, Alexandria, West Dunbartonshire, at around 2.30pm on Saturday.
It was initially described as a “disturbance” and Police Scotland said it is continuing to investigate the full circumstances.
It is believed all five had taken prescription medication along with alcohol and other drugs.
Officers are appealing for information on the illgal distribution of prescription medication in the area.
Two of those who fell ill were taken by ambulance to the Queen Elizabeth University Hospital in Glasgow while the others are being treated at the Royal Alexandra Hospital in Paisley.
Detective Inspector Andy Doherty said: “Inquiries are ongoing to establish the full circumstances and how these people came to be unwell, however at this time we believe all five had taken prescription medication mixed with a quantity of alcohol and other drugs.
“I would like to reassure the local community that there was no violence involved in this incident.
“Anyone with information regarding the illegal distribution of prescribed medication in the West Dunbartonshire area is asked to contact Clydebank CID via 101.”