A prison where an inmate was allegedly used as a "guinea pig" for psychoactive drug Spice had no rules to deal with those suspected of taking the substance, a report found.
Shabol Ahmed, 33, was found dead at HMP Highpoint in Suffolk in July 2016 a day after collapsing on the exercise yard.
The Prison & Probation Ombudsman said staff were "unclear" how to manage a prisoner under the influence of Spice.
The Prison Service said it had "taken action" after Ahmed's death.
Ahmed, who had learning difficulties and was serving a sentence for grievous bodily harm according to the Ombudsman, was found dead in his cell early in the morning on 19 July 2016.
The report said the month after Ahmed died bosses at the jail implemented a strategy to tackle new psychoactive drugs.
He had collapsed in the exercise yard at 18:25 BST the previous day and another inmate said Ahmed could have been given a cigarette to test a batch of Spice, according to the report.
Inmate Zia Islam told an inquest, which ended earlier this month, that some "thought it was funny just to watch people overdose" on Spice and that "some people were being used as guinea pigs".
Ahmed was returned to Highpoint later that evening but the report said the hospital did not provide any further instructions for his management.
He was found dead in his cell the next morning but a post-mortem into his cause of death was inconclusive.
A toxicology report found it was "possible that [new psychoactive substances] contributed to Mr Ahmed's death due to possible chronic damage to the heart".
The Ombudsman said: "We found [jail] staff were unclear on the immediate steps they should take to manage a prisoner who they suspect is under [new psychoactive substance] influence."
The inquest jury recorded an open conclusion but found there was "a serious failure in the sharing of information between the prison, healthcare and the hospital".
They said there was "inadequate training within the prison to deal with drug-related incidents and their aftermath".
A Prison Service spokeswoman said: "We have already taken action to prevent this happening again, including a full review of healthcare and the drug recovery unit.
"We are carefully considering the findings in the report and will take further action where necessary."