ON A busy street corner in Manchester’s central shopping area, a young man has just collapsed, unconscious. Judging by his grubby clothes, he is one of the many people sleeping rough in the city centre. There is no need to call an ambulance, says a shop assistant, after assessing the situation. “It’s spice,” he explains with a shrug, as he walks back inside, adding that it would be best to stay away, because when the man comes round he may become violent.

Spice is the name collectively given to 200-300 synthetic cannabinoids, drugs that hit the same brain receptors as cannabis but are more potent and addictive. The drugs, made mostly in China and illegal in Britain, take the form of chemicals sprayed onto dried plant leaves and smoked.

In 2017-18 only 0.4% of 16- to 59-year-olds in Britain used the category of drugs that includes spice, according to the Home Office. But spice has become an epidemic among two groups not covered by these statistics: prisoners and rough sleepers. Over 90% of homeless people in Manchester smoke it, according to one survey, as do many in other cities, including Birmingham, Bristol, Leeds and Newcastle. It is “one of the most severe public-health issues we have faced in decades,” wrote 20 police commissioners in an open letter to the Home Office last month.

The trouble is that what has been tried and tested for other illegal drugs cannot be readily copied for spice. For a start, its effects on users are unpredictable. One reason is the rapid turnover of the chemicals in the mix. Chinese authorities have been banning individual chemicals found in spice, but the laboratories that make them get round the bans by tweaking the composition of their product.

Another worry with spice is that the spraying of the chemicals is uneven, leading to highly variable potency within the same batch. In April last year the concentration of chemicals in spice in Manchester jumped from 1-2% to nearly 20%—possibly because someone missed a decimal point in a recipe found online, says Robert Ralphs, a criminologist at Manchester Metropolitan University. Ambulance crews were overwhelmed, with nearly 60 call-outs for comatose people on the streets in a single day. Smaller spikes in concentration have turned users into what the tabloids call “spice zombies”, for their pale faces, pink eyes and staggering gait.

Doctors and paramedics are having to learn on the fly how to treat severe reactions to the many varieties of spice. Psychosis and paranoia are common, which is why users are often aggressive. One hospital doctor, who sees someone high on spice on almost every shift, says that the effects are wildly varied and that it is impossible to predict how long they may take to wear off. One man on spice walked around the ward naked for three hours. “We didn’t know what to do,” the doctor says. “We just locked the door, locking ourselves in with him.”

A national network set up last year collects clinical reports about spice users brought to hospital emergency departments. The process is similar to that used to track adverse reactions to medicines. Treatment guidelines are updated online.

Prisons are also grappling with new problems caused by spice. Failing a drug test while inside or on parole brings extra time behind bars. But the prisons’ drug-testing kits do not detect synthetic cannabinoids, so many drug users switch to spice in order to hide their habit. “You go in as an alcohol, heroin or crack user and come out as a spice user,” says Mr Ralphs.

Peter Morgan, who has worked with vulnerable youths in Manchester for 20 years, says spice has been a “horrific thing” for the homeless. He lays out the problems in “The Spice Boys”, a book about a group of young homeless people hooked on the drug. By making users limp, spice turns them into targets for theft, rape and assault. Outreach workers can usually catch four or five hours of lucidity a day from a heroin addict. With spice, the brain is foggy all the time. “You need to smoke it constantly,” says one former user.

Weaning people off spice is also tougher than getting them off other drugs. Some do not consider themselves addicts, a designation they reserve for heroin junkies. Even as they struggle with withdrawal symptoms and resort to selling sex or stealing to get their next fix, they see spice as not much more harmful than cannabis. So far nothing makes an effective substitute for it, as methadone does for heroin. Treatment therefore targets withdrawal symptoms, using drugs that dull pain, stomach problems and psychosis.

One thing that those who pick up spice tend to have in common is previous drug use. As spice users become more stigmatised, those on other illegal drugs may be less inclined to switch to it. Even some heroin users are now looking down on spice zombies, says Mr Ralphs.