Concerns have been raised about the use of Spice in Sheffield city centre, with the drug causing people to collapse or freeze.
Both South Yorkshire Police and the Archer Project, which supports the homeless in Sheffield, claim it is popular among those on the streets.
The addictive and fast acting drug, which was once a legal high, is sprinkled into cigarettes and often shared between groups, lowering the cost for users.
Tim Renshaw, chief executive of the Archer Project, said the charity is so concerned about the use of the drug, users caught with it or high on it are banned for a week.
He said Spice is 'incredibly popular' with rough sleepers and that Archer Project staff and volunteers see people under the influence daily, claiming they either freeze 'as though they are mummified', display signs of having a seizure, collapse or appear delirious.
Two ambulances were recently called to the charity's Campo Lane base in one day to treat users who suffered adverse reactions.
"The problem with Spice is that it is cheap and a very social drug - people sprinkle it into cigarettes and smoke it, often sharing it with others, which results in a number of people under the influence very quickly and at the same time. We see people high on it daily," he said.
"It also doesn't smell like cannabis, making it more difficult to detect those using it in public.
"The chemical make-up of it is changing all the time, making its effects unpredictable. In fact there are seasoned drug users who think of Spice as the ultimate evil and these are people who have taken every drug going over the years.
"It is dangerous because of its immediate effect, its unpredictability and its addictive properties."
Inspector Neil Mutch, of South Yorkshire Police, said: "While the psychoactive substance Spice is not being recovered or seized in significant quantities, as you might find with other more established illegal substances, it is causing concern among police and other agencies who work closely in partnership to keep the city centre safe.
"Though we do not have measurable data to confirm this, what we're experiencing anecdotally is an increase among vulnerable individuals using Spice. These individuals may be choosing to sleep rough, or they may have existing substance abuse issues, and require support from specialist organisations. Police and other agencies find this increase worrying because it could place those individuals at even greater risk of harm.
"Locally in the city centre, we have seen that individuals using Spice are engaging in more anti-social behaviour and associated low level disorder, which we are working hard to combat in partnership with our partners in the council and health.
"We're also determined to identify any people who may be involved in the distribution of this substance to others, which in addition to possession is a criminal offence and can be prosecuted under the new legislation.
"It's important that vulnerable people are able to seek support from the appropriate agencies, but police will also respond robustly to any incidents of crime or any information about those individuals involved in the now-illegal psychoactive substances trade.
"If anyone living or working in the city centre does have information that could help our officers identify dealers and tackle this issue, please do get in touch."
A spokesman for the Sheffield Drug and Alcohol and Domestic Abuse Coordination Team said: "We know that there has been an increase in the number of people taking this potentially very dangerous drug, and would advise anyone considering using this substance not to do so.
"For those who are using Spice, we would like to reassure them that there is help available. Support and treatment is available for anyone in Sheffield experiencing drug and alcohol issues. All services in Sheffield take self-referrals and are open access, meaning people can walk in and be seen that day."