A 46-year-old male nurse has been arrested by Greater Manchester Police investigating the poisoning of patients at Stepping Hill Hospital, in Stockport.
Assistant Chief Constable Terry Sweeney said the man was only being spoken to in relation to the tampering of medical records which led to one patient being administered additional amounts of medication.
"It is due to the diligence of staff at the hospital that we were made aware about the potential tampering of medical records and everyone should be reassured that, like the hospital, we initiated a swift response and have consequently arrested a member of staff in relation to this matter," he said.
"I must stress that at this time he is only being spoken to in relation to these matters and, as we have previously said, we will not and cannot rule out making further arrests in the coming days or weeks.
"I cannot stress enough just how complex an investigation this is and it is one that we continue to commit significant resources to."
At least 17 people were poisoned by the contamination of saline solution at the hospital last year. Among that number are the cases of Tracey Arden, 44, Arnold Lancaster, 71, and Alfred Derek Weaver, 83, who all died.
The alarm was first raised by hospital staff on 12 July when a higher than normal number of patients were reported to have "unexplained" low blood sugar levels amid fears that saline solution had been contaminated with insulin.
Charges that she tampered with saline solution with intent to endanger life were dropped by the Crown Prosecution Service (CPS).