Disgraced publicist Max Clifford was given wrongly-labelled medication at least twice in prison before he died, a report has found.
Clifford, 74, died two years ago while serving an eight-year sentence at Littlehey Prison in Cambridgeshire for historical sex offences.
The Prisons and Probation Ombudsman said inmates should be monitored more closely to ensure medicine was taken.
The prison's healthcare provider said it had implemented a plan to improve.
The report, written in November 2018, was published after an inquest into Clifford's death concluded he had died of natural causes.
On one occasion Clifford became dizzy after taking a mislabelled tablet four times the intended strength, the report said.
He told staff he realised their error because of the effects of taking the 8mg tablet of a heart medication that was labelled as 2mg.
After Clifford's death, "a significant quantity" of prescription medicine was found unused in his cell, said deputy ombudsman Richard Pickering.
"We are concerned that Mr Clifford was given incorrectly labelled medication on at least two occasions, and that staff did not monitor his medication compliance adequately at other time," he wrote.
He also raised concerns at a delay in transferring Clifford to hospital, because staff called an urgent ambulance instead of the emergency one the GP requested.
The report recommends that prison pharmacy services be reviewed to avoid medication errors, staff ensure patients take their medication and respond with greater urgency to emergency situations.
Clifford died on 10 December 2017 at Hinchingbrooke Hospital, near Huntingdon, where he was taken after collapsing in the shower.
His inquest heard he could have received better care if his heart condition had been diagnosed sooner.
A post-mortem examination recorded the cause of death as congestive heart failure.
Northamptonshire Healthcare NHS Foundation Trust, which provides healthcare at HMP Littlehey, said the coroner had accepted its action plan and evidence of implementation.
It said it would "continue to share learning and implement our findings to ensure our care is of a high standard".