The Royal Bar & Hotel in Marine Road Central saw a number of guests collapse due to a suspected carbon monoxide leak on Boxing Day morning.
Officers evacuated the building at around 9.45am and a cordon was put in place while gas service engineers worked to make the area safe.
"Five people were taken to hospital with symptoms of carbon monoxide poisoning," a spokesman for Lancashire Police said.
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"They have all since been discharged."
Two fire engines from Morecambe also attended the scene and ventilated the building after being contacted by the ambulance service.
A spokesman for Lancashire Fire and Rescue Service said: "The ambulance service treated several people at the scene while our crews remained at the scene for around six hours working with Cadent Gas to make the property safe.
"A number of people were taken to hospital.
"The investigation has now been passed to Lancaster City Council."
Lancaster City Council said they supported the emergency response and were working with their "partner agencies to determine the exact circumstances of the incident".
Lancashire Police asked anyone with information to call 101, quoting log reference number 0353 of December 26.
North West Ambulance Service and The Royal Bar & Hotel have been approached for comment.
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