Revellers were evacuated from a Kettering town centre bar after a gas canister was set off in the early hours of Sunday morning (November 30).
More than 100 people were said to be at Bar Qbano, in Ebenezer Place, whe the incident happened.
East Midlands Ambulance Service (EMAS) said its fast response vehicle was on the scene within seven minutes and was followed by two more ambulances.
A spokesman for EMAS said: “Three patients required treatment for breathing difficulties and abdominal pain, two of those were treated on scene and one patient was taken to Kettering General Hospital.
“Our Hazardous Area Response Team have specialist knowledge in chemical, biological, radiological or nuclear incidents, they were not required to attend but provided over-the-phone support to the emergency services on scene.
“Our second DCA remained at the club to support police and fire and be on hand to assist anyone needing medical support.”
A spokesman for Northamptonshire Police said the force was looking for witnesses.
He added: “At about 2.20am on Sunday, an unknown substance, believed to be a CS-type gas, was released on to the dance floor of the busy club.
“The club was evacuated by staff and a number of people suffered slight breathing difficulties were attended by paramedics at the scene.
“Officers investigating the incident are asking anyone who was at the club and who may have witnessed the release of the substance or seen any suspicious behaviour, to contact them.”
Witnesses, or anyone with information, should call Northamptonshire Police on 101 or Crimestoppers on 0800 555111.