A prison where an inmate died has been told to avoid housing mentally ill inmates in segregation and to tackle the issue of illicit drugs.
Charlotte Nokes, 38, who had been on suicide watch, was found dead in a cell at HMP Peterborough in July 2016.
She was more than eight years into an indefinite prison term.
An inquest ruled she died of natural causes, but an ombudsman raised concerns about her "long periods" on a segregation unit.
The ombudsman also said "no action" was taken when she seemed over-sedated.
Private company Sodexo, which runs the prison, has been contacted for comment.
Nokes, from Hayling Island, Hampshire, died on 23 July 2016 after being found unconscious in her bed.
She was jailed for 15 months in 2008 under an Imprisonment for Public Protection (IPP) sentence for possession of an offensive weapon and attempted robbery.
An inquest heard she suffered sudden arrhythmic death syndrome (SADS) in her cell.
An ombudsman report said she received good mental health care at the prison for conditions including anxiety, but it was not clear if her "significant period of time in segregation" had been necessary.
It also said staff took "no action" when she reported using illicit substances and appeared to be heavily sedated.
Prisons and Probation Ombudsman Nigel Newcomen said: "Even without evidence of any bearing on her death, I have some concerns over the prison's extensive use of segregation for Ms Nokes and the response to her apparent illicit substance abuse, including the need for better sharing of information between healthcare and discipline staff."
He recommended that prisoners at risk of suicide or self-harm should not held in the segregation unit unless all other options have been considered and said there needed to be a strategy to reduce the availability of illicit drugs .