The 27-year-old, from York, was arrested yesterday over alleged dealing in the Warehouse nightclub in North Bridge Road in the early hours of Saturday, May 14.
South Yorkshire Police said officers were called to the club after two men were taken ill.
A 32-year-old man, who flagged officers down with chest pains after visiting the club earlier that night, was taken to hospital for treatment. He was later discharged.
A 29-year-old man, who is believed to have taken a tablet, currently remains in hospital in a critical and life threatening condition.
The force has issued a photograph of tablets similar to what the two men are believed to have taken in the club.
Detective Sergeant Andy Knowles said: “The investigation into this matter remains ongoing and we are keen to find out where these tablets came from and who was in possession of them.
“I’d ask you to cast your minds back to that night, were you in Warehouse? Did you see anything suspicious? If so, I’d ask you to get in touch with us.
“If you’ve bought pills similar to these, do not take them as they could be extremely harmful. If you have, please seek medical assistance as soon as you can.”
Anyone with information should call South Yorkshire Police on 101 or Crimestoppers on 0800 555111.