BRITISH tabloids call them “spice zombies”. In America they make headlines for overdosing on “fake weed”. In other countries they are users of “legal highs” gone too far. They are whisked to emergency rooms after collapsing or falling into a stupor, in numbers that often spike sharply for a day or two. The drugs to blame? Synthetic cannabinoids, chemicals that hit the same brain receptors as cannabis but are much more potent and addictive. They are made mostly in China, shipped as powders and sprayed onto dried plant leaves, so that they can be smoked. Spice, the collective name given to the drugs in Britain, is “one of the most severe public-health issues we have faced in decades,” wrote 20 of the country’s police commissioners in an open letter earlier this year.

The biggest problem with spice is that its effects on users are unpredictable. One reason is the rapid turnover of the chemicals in the mix. There are several hundred known synthetic cannabinoids and new ones are relatively easy to concoct. 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. Spraying of the chemicals can be uneven, leading to highly variable potency within the same batch. Other missteps can also wreak havoc. Last year, the concentration of the chemicals in spice sold in Manchester suddenly jumped ten-fold, possibly because someone missed a decimal point in a mixing recipe. As a result of all this, the effects of spice and their duration can vary wildly, complicating matters for paramedics, hospitals and the police. Paranoia and psychosis are common, making some users violent.

Moreover, weaning people off spice is tougher than getting them off other drugs. For a start, some of those hooked on spice do not see themselves as addicts, thinking that it is not much more harmful than cannabis. In America, which, unlike Britain, has banned only some synthetic cannabinoids, such views may be partly due to spice variants being sold in shops as herbal incense. Outreach workers who help homeless addicts face another problem. They can usually catch four or five hours of lucidity a day from a heroin addict. With spice, however, the brain is foggy all the time because addicts tend to chain-smoke it. And so far nothing makes an effective substitute, as methadone does for heroin. Treatment therefore targets withdrawal symptoms, using drugs that dull pain, stomach problems and psychosis.

For now, spice is less popular than other street drugs. In 2017-18 only 0.4% of 16- to 59-year-olds in Britain used the category of drugs that includes spice. In America, 7% of high-school students have tried spice, whereas 36% have tried cannabis. But in many cities in Britain and America spice usage has become epidemic among the homeless, who in many cases will already have drug addictions and will be tempted by something that is cheap and can apparently make the passing of two days feel like two hours. Spice is also replacing other illegal drugs among those whose careers or liberty depend on clearing a drug-screening test. Standard drug tests do not detect synthetic cannabinoids, so users of cannabis, heroin or cocaine who are in prison, on parole or in the army switch to spice to hide their habit. But the surprises that spice's chameleonic recipe can throw up make that a risky strategy.