One thing is certain in Freetown, Sierra Leone’s capital, says 18-year-old Abdullah: you are never far from a kush dealer. Some call its impact the “kush devil”. For Abdullah, addiction resulted in him dropping out of school and begging on the street. Now he is staying at the City of Rest Rehabilitation Centre, one of Sierra Leone’s only rehab facilities.

A single joint of kush can cost 5 leones (20p), which can add up quickly when buying multiple joints a day – in a country where youth unemployment stands at 60%, one of the highest in west Africa.

Looking at the floor, Abdullah says he hasn’t used kush for the two weeks he has been at the rehab centre – but it’s everywhere. “Anywhere you go you see kush, there’s more dealers than users.”

He alleges that even some in the police are in on the trade in Sierra Leone. Almost all the centre’s 16 patients are struggling with kush addiction, yet for a drug that appears so plentiful in Sierra Leone, its origin and composition remain a mystery.

Authorities have likened kush to a synthetic cannaboid product – akin to the so-called “zombie drug” spice – that can be smoked, but no one appears to be sure what’s in it. Kush users and the doctors treating them have suggested there are dangerous potential ingredients including fentanyl and tramadol, and also formalin, a chemical most often used to preserve human bodies. Shoe polish is another mooted element.

Many people who spoke to the Guardian cited horrifying stories of human bones being crushed up into kush, with local media reporting arrests for gravedigging for this purpose, although this has not been verified.

The rehab centre, a single-storey concrete block in one of Freetown’s hilly suburbs, offers an initial 90-day programme, with medication for detoxification from the drug as well as counselling, and an aftercare plan to deal with potential relapses. But part of the building lies in rubble after a fire partially gutted it a few months ago, drastically reducing capacity.

I smoked it to feel a release. But it made me lose myself. It’s the worst problem currently in Sierra Leone

“Kush destroyed my life,” says 21-year-old Tejan, who stopped using two weeks previously. “I smoked it to feel a release, but it made me lose myself.”

Tejan used the college lunch money his parents gave him to buy kush, which was easy to find as there was a dealer behind his house. “It’s the worst problem currently in Sierra Leone,” he says.

Augusta Johnny, the centre’s programme co-ordinator, has seen a “drastic” increase in use of the drug in the past five years. It appears to be a “profitable business”, she says, although no one is sure who is ultimately profiting.