Police investigating the 39-year-old man's collapse at The Arches on Saturday night said inquiries are ongoing.
It is thought he took four ecstasy tablets at once after being searched and confronted about the pills by staff.
He collapsed and was rushed to Glasgow Royal Infirmary, where doctors are understood to have put him in an induced coma in intensive care.
It is not clear if he took Mortal Kombat pills, the type linked to the death of 17-year-old Regane MacColl in February.
A Police Scotland spokeswoman said: "Around 11.55pm on Saturday March 29, police were called to a report of a man taken ill after taking illegal drugs while being searched in the queue for the Arches nightclub.
"The 39-year-old was taken by ambulance to Glasgow Royal Infirmary where he is in a serious but stable condition."
Club bosses recently introduced a policy of restricting admission to its club nights to over-21s, with promises to ramp up security and searches.
Despite police calls for its licence to be reviewed, The Arches faced no action after Ms MacColl's death.
Ms MacColl, from Clydebank, West Dun-bartonshire, is believed to have taken a Mortal Kombat pill at The Arches Club before she died.
Police and health officials warned about the dangerous red pill featuring the image of a dragon after it was linked to her death.
Dr Neil Howie, NHS consultant in emergency medicine, said: "We cannot stress highly enough that anyone who takes an illicit drug is putting their health, and indeed their life, in jeopardy."
Last month, the Evening Times revealed police sniffer dogs are being used to combat deadly drugs in city nightclubs.
Fourteen clubbers were reported to the procurator fiscal following the operation at The Arches in one weekend.
We told last week how the number of drug arrests have soared since the death of Ms MacColl.
More than 150 people were arrested in the six weeks after the her death on February 1.