A healthcare professional at a hospital has been arrested over the alleged administration of poison or a noxious substance to patients.
Blackpool Teaching Hospitals NHS Foundation Trust raised concerns over care to some patients on 8 November, Lancashire Police said.
The allegations related to the alleged mistreatment and neglect of some patients on the stroke unit in November, police said.
She has been bailed until 6 December.
The worker has also been suspended from duty.
'Reassure all our patients'
The BBC understands officers are investigating the alleged incorrect administration of drugs.
Det Ch Insp Jill Johnston said: "The inquiry is complicated and we have a team of specialist detectives working on this case who are also offering support to those families who have had loved ones identified as potentially being involved.
"Our priority and the priority of Blackpool Teaching Hospitals NHS Foundation Trust is to ensure the safety of patients. We are working closely with the trust as part of the investigation."
Wendy Swift, chief executive of the trust said in a statement: "We take all allegations of this type extremely seriously and when the concerns were raised we immediately contacted the police."
She said the trust was co-operating fully with the police investigation.
"We would like to reassure all our patients that all necessary actions are being taken.
"The hospital remains a safe and caring environment for patients and the provision of services will continue as usual.''