They were admitted over the weekend when they became ill after taking drugs, sparking a warning from police.
Seven men and two women became seriously ill in Glasgow city centre and were taken to hospital between 1am and 4am on Saturday.
They are aged 17 to 22, according to a statement from Strathclyde Police. It is understood those involved had been in at least four city-centre venues, including the Arches, Sub Club, Saint Jude's and Hummingbird.
Eight were admitted to the Glasgow Royal Infirmary and one to the Western Infirmary.
They were all agitated and showed signs of high blood pressure, fevers and seizures.
Superintendent Kirk Kinnell of Strathclyde Police said: "We are taking this matter extremely seriously and are currently carrying out extensive inquiries to establish where these drugs have come from.
"Searches are being carried out at a number of venues in connection with this inquiry.
"Taking any type of illegal drug is dangerous. Often these substances contain a cocktail of ingredients which can vary from pill to pill. The effects can be unpredictable and can cause serious harm."
Police have increased patrols in the city centre because of the problem, and officers will speak to club and door staff to offer advice and gather information.