"I thought it would kill the time. I had to smoke it, just so I wouldn't feel ill."
Jason first took Spice when he was in prison, now he has to smoke it every hour or he suffers withdrawal symptoms.
He spoke out as a homeless charity said Spice use was now at "epidemic" levels in Cardiff , with many of the city's homeless population now using the drug.
"People don't want the heroin any more, they want the Spice," he said.
"They turn to Spice and it ruins their life even more. Coming off Mamba (Spice) is worse than coming off heroin, you feel suicidal," said Jason, who was released from prison in April.
Richard Edwards, chief executive of homeless centre The Huggard Centre in Cardiff, said when Spice arrived in Cardiff two years ago it was "like the day of the apocalypse".
At the last official count in Cardiff there were 53 rough sleepers in Cardiff although the population does fluctuate.
Mr Edwards estimates nearly 70% of the city's homeless population are now using the synthetic drug.
The former "legal-high" is a psychoactive substance, designed to mimic the effects of cannabis, but can have severe debilitating effects.
It can leave users in a Zombie-like state, and images of people apparently under the influence of the drug in Welsh towns and cities have been shared on social media.
"On one hand you have something which will subdue somebody and have a similar effect to cannabis, but on the other hand you've something akin to heroin, so it's highly addictive and can put someone in respiratory arrest if they overdose on it," said Mr Edwards.
Jason, now homeless, said he could arrange for a £15 bag of the drug to be delivered to St Mary's Street within minutes.
"It's the craziest drug I've ever been on," he told BBC Radio Wales, adding he took the drug to dull the pain of being on the streets and while he knew the risks he could not stop.
"People come up and they chuck a bit of change and think it's rosy," he said. "It's not all rosy.
"People smoke it 'cause they want to blank out what's going on in their lives anyway.
"Some users slip into a trance like state which can last for minutes or hours, when they come round they're often angry, disorientated and frightened."
Jason's friend Ed has been a habitual drug user since his early teens and now uses Spice, which he says is "cheap, cheerful and is readily available".
"It gives you that feeling of euphoria like heroin," he said.
"The attraction of the Spice is that I'm sitting here in the middle of town next to you, rolled a spliff and I'd probably be able to sit on the end of this bench and smoke it.
"Unless you knew the smell you'd have no idea. With heroin and crack you need paraphernalia - foil to smoke the heroin, a crack pipe to smoke the crack, if they're injecting the heroin, the needles, they need all the kit to do that. I couldn't exactly set up that equipment in St Mary's Street."
He and Jason try to work out how many times they've "gone under" - street speak for lapsing into unconsciousness.
"I could smoke a pipe now," said Jason. "I could go under and have a heart attack. Simple as that."
Mr Edwards said the use of Spice had reached epidemic proportions in Cardiff.
But Lindsay Cordery-Bruce, chief executive of the homeless charity The Wallich, stressed the problem affected a small and troubled community.
"We need to stress that alcohol is still killing more people than all the illegal drugs combined together," she said.
"We need to make sure we don't scare the life out of the public. Spice isn't grinding Wales to a halt."