Nine people have been admitted to hospital after taking drugs, sparking a public 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 between 17 and 22 and are believed to have taken a "combination of tablets and powder", according to a statement from Strathclyde Police.
Of those affected, eight were admitted to Glasgow Royal Infirmary and one to the Western Infirmary with similar symptoms.
They were all reported to be agitated, showing signs of high blood pressure, high fevers and seizures. Police became involved due to the high number of people admitted to hospital over a short period of time.
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 as result of the incident, and officers will speak to club and door staff to offer advice and gather information.
NHS Greater Glasgow and Clyde public health consultant Anne Scoular said: "Ingesting club drugs is always risky as you can never quite be sure what you are taking. The result can be very unpredictable, especially when combined with alcohol. Symptoms include hallucinations, severe agitation and aggression, high heart rate and seizures.
"The situation last night saw some quite heightened symptoms among a number of people which we would not normally see. While we urge people not to take these high risk club drugs, if you do and you or your friends show any signs of the symptoms above, I would urge you to seek medical advice immediately."