Murder squad detectives investigating suspicious deaths at Blackpool Victoria Hospital are probing whether potential victims were given prescription-only sleeping pills.
The drug is known as Zopiclone, and is a prescription-only drug that comes in pill or liquid form.
Nursing staff administered the drug to keep elderly stroke patients heavily sedated during night shifts, it was understood.
According to the NHS, it has to be ordered specially by a patient's doctor.
It has been claimed up to eight former patients on the stroke unit have so far been identified as potential victims.
Three nurses have been arrested on suspicion of poisoning patients and suspended from duty.
They have all been released on bail until early next year after being quizzed under caution.
The original allegations were made by a whistle blower, who is also a member of staff of the 780-bed hospital.
The hospital called in the police on November 8, with the first nurse arrested the day after.
Earlier this week, heartbroken families said they were worried their family members have been caught up in the investigation.
One grieving daughter was told to cancel her dad's funeral as his body was taken away for a post-mortem examination.
The matter has been referred to the Nursing and Midwifery Council, while police have also been looking at the way drip feeds and canulars were used on the ward.
WHAT IS ZOPICLONE?
Zopiclone is a type of sleeping pill that can be taken to treat bad bouts of insomnia. It helps you fall asleep more quickly, and also helps stop you waking up during the night.
Zopiclone comes as tablets. It also comes as a liquid for people who find it hard to swallow tablets, but this has to be ordered specially by your doctor.
This medicine is only available on prescription.