Paul Stokes, 39, was waiting to get into the Arches in Glasgow when he collapsed just before midnight on Saturday.
It is the same club where schoolgirl Regane McColl collapsed and later died after taking a pill.
It is thought gym enthusiast Mr Stokes was being searched by staff when he fell to the ground.
The emergency services were called and he was taken by ambulance to Glasgow Royal Infirmary where his condition has been described as 'stable'.
Mr Stokes, of Motherwell, North Lanarkshire, has been put into a coma by medics, according to friends.
It is believed he had recently come out a long term relationship and his drug taking is said to have been "out of character".
Mr Stokes is a regular user of the Ravenscraig gym in Lanarkshire and lists his personal interests as being a member of the Medicaid First Aid course and the Slow Cooker Appreciation Society on his Facebook profile.
The incident comes soon after 17-year-old Regane collapsed in the Arches and later died in hospital.
The Arches scrapped all its club nights for two weeks following the tragedy and was investigated by the authorities. Police Scotland set out a series of recommendations for the Arches during the meeting of Glasgow City Council's Licensing Board, including mandatory searches of all customers, and 'moments of calm' during club nights.
A Police Scotland spokeswoman said: "The 39-year-old was taken by ambulance to Glasgow Royal Infirmary where he is in a serious but stable condition."
The hospital said Mr Stokes was stable.