Charges have been dropped against a nurse accused of contaminating saline at Stockport's Stepping Hill Hospital.
Rebecca Leighton, 27, of Heaviley, was arrested by police investigating the deaths of several patients in July.
Greater Manchester Police have revealed they are investigating 40 cases of contamination including seven deaths.
Prosecutors said there had not been "sufficient evidence" for whether the case could go ahead. Miss Leighton said she had been "living in hell".
Officers said Miss Leighton was released from prison on Friday afternoon after she was told by the governor at HMP Styal in Cheshire that the charges against her were being dropped.
They said police were in contact with her to guarantee her safety and "to try and help her back into the community".
The Crown Prosecution Service said it was "no longer appropriate" to continue the case against Ms Leighton.
Solicitor Carl Richmond read a statement on behalf of Ms Leighton in which she thanked her family, friends and members of the public who "never doubted my innocence during this living nightmare".
It read: "I've been in a living hell and was locked up in prison for something I had not done.
"It was so frustrating for me knowing that the person who actually carried out these terrible acts is still out there.
"My life has been turned upside-down. All I ever wanted to do was to pursue a profession in nursing and care for my patients."