Shabul Ahmed, who had learning difficulties, collapsed after using the synthetic cannabis in the exercise yard at HMP Highpoint near Haverhill, Suffolk, a hearing in Ipswich was told.
The 33-year-old was taken to hospital on July 18 2016 and returned to prison later that evening.
He was found dead in his cell at 6.25am on July 19 after roll call, Suffolk area coroner Jacqueline Devonish said.
She said Ahmed was "suspected to have consumed an illicit substance, Spice" before he collapsed at around 6.25pm on July 18.
Fellow inmate Zia Islam, who said he spent seven months in the cell next to Ahmed, said: "When Spice comes on people thought it was funny just to watch people overdose.
"Some people were being used as guinea pigs."
Asked by coroner Ms Devonish if Ahmed was ever a victim of that, he replied: "Yes, that's what I think happened that day on the 18th."
Asked why he thought this was the case, he said: "The amount of Spice that was used in that spliff, the amount of damage it caused.
"If someone was smoking it for pleasure there's no way they would have used as much and as powerful.
"I saw people take two puffs and within 10 seconds they were on the floor."
He said a cigarette containing Spice was passed around a circle of prisoners in the exercise yard on July 18.
Asked by the coroner if Spice was readily available in the prison, he replied "yes".
He said Ahmed had been convicted of robbery and "just wanted to do his time".
"He spent all day watching cookery shows and Tipping Point," he said, adding Ahmed did not have other friends in the prison.
The inmate said he did not believe Ahmed used Spice in his cell after he returned from hospital as he did not have a lighter.
Ahmed's sister Nazma Begum said her brother was "caring, loving, would look after his younger sisters" and had previously worked in restaurants.
But he was "never the same person" after he was jailed for the first time aged 18, she said.
She said her brother's arms showed signs of heroin use after he was released but he refused to talk about what happened.
He was jailed on two further occasions, she said.
"I believe my brother was taken advantage of, picked on, used for the way he was really, not being able to read or write," she said.
The inquest, listed for seven days, continues.