The suspects – one man and three women – have today been detained in connection with allegations of mistreatment and neglect at Blackpool Victoria Hospital.
Concerns were first raised to police by Blackpool Teaching Hospitals NHS Foundation Trust in November last year.
The investigation led to the arrest of a woman in November and a man and a woman in December.
The trio – also all healthcare professionals – have since been released under investigation.
All seven are accused of administering a poisonous or noxious substance with the intent to injure and ill treatment or wilful neglect.
Press reports suggest nurses and healthcare assistants are among those arrested.
The allegations relate to care provided on Blackpool Victoria Hospital’s stroke unit in November 2018.
The first three individuals arrested were suspended from the trust in November while the latest four have been suspended since December.
Lancashire Police is working with HM Coroner for Blackpool and Fylde and a number of post-mortem examinations have been conducted as part of the inquiry.
In January, detectives confirmed a murder investigation had been launched following the death of a woman in her 70s on the stroke unit in November, but this is being treated as a separate inquiry.
Detective Chief Inspector Jill Johnston, of Lancashire Police, offered reassurance.
“I understand further arrests could be concerning but I would like to reassure people we have a dedicated team of detectives working on this investigation,” she said.
“Our priority and the priority of Blackpool Teaching Hospitals NHS Foundation Trust is to ensure the safety of patients,” Ms Johnston.
She added that the investigation remained “complex and sensitive” and that no timescales had been set.
Police were supporting families caught up in the incident, Ms Johnston added.
Anyone with information is asked to call police on 101 quoting log reference 0553 of 14 November.