It's called "frozen spice", and it's becoming a problem of almost epidemic proportions among Manchester's homeless.
Spice is a chemically made synthetic marijuana, and like many of the drugs categorised as New Psychoactive Substances, or NPS, it used to be sold as a so-called legal high.
Since May 2016 drugs such as spice have been illegal, but that doesn't mean that, in Manchester at least, it can't be easily obtained.
Shaun and Jason first started smoking spice when they were in prison. Their addiction means that most of the money they make begging in the city centre is given to dealers to buy bags of the drug at £10 a time.
"The first time I had it in prison it had me up against my cell wall for 24 hours," Shaun tells us.
"I said I'd never do it again after that, but I've been smoking it ever since.
"I'm going to give it up," he promises himself. "But the side effects and withdrawal can be worse than heroin."
Jason says he too wants to stop smoking spice.
"The first thing I need to do it is accommodation," he says.
"I'm moving into a hostel soon, so maybe that'll be my chance to change this lifestyle, 'cos that's what this is, it's a lifestyle."
As we spoke to the pair, the spice they were smoking began to take effect, and they lapsed into a morbid silence.
It's a scene all too familiar to Julie Boyle. She works for the LifeShare Charity in Manchester, and estimates more than 95% of the homeless people she helps are using spice.
"This latest variant, frozen spice, is really bad. I've never seen anything like it," she tells us.
"It leaves them almost catatonic when they use it. It's like the Walking Dead in the centre of town now. I saw one man so out of it, two other men were just going through his pockets.
"It's difficult to tackle too," she adds. "The addiction is so severe that it's actually harder to come off spice than it is to come off heroin."
Inspector Phil Spurgeon, from Greater Manchester Police's City Centre Team, said: "We are working hard to prevent the dealing of spice in the city centre, focusing on building intelligence and carrying out enforcement work.
"We are seeing more and more people using spice in busy public areas in the city centre and we must tackle it to ensure the area is still a safe place for the whole community."
City councillor Paul Andrews said: "We are aware there is a growing problem with New Psychoactive Substances (NPS) including spice nationally, particularly among vulnerable groups and homeless communities nationwide, including Manchester.
"This current strain of spice seems to be causing very specific side effects and we are working with our partners to try to find out what is in it."
The ambulance service say the growing number of spice-related incidents is straining their resources.
With suppliers constantly working on new variants of the drugs to make them stronger and more addictive, the fight to keep them off Britain's streets is getting harder.