The potential triggers for relapse are infinite. "It could be a major life event or it could be a seemingly insignificant thing," Tyrell tells me. "It can even be a sensation at the back of your mind; an itch that you really want to scratch." Speaking to fellow addicts, it becomes clear that their experiences vary wildly. Lily, 40, relapsed after eight years of sobriety when her sister was diagnosed with breast cancer. Thirty-six-year-old Shane relapsed after losing his house and his job, and moving into a dilapidated squat. For my friend Hannah, though, the trigger was nothing so large. There’s a saying in recovery circles: "If you hang around a barbershop long enough, you’re going to get a haircut." "For me, it was just that," she tells me. "I'd been clean for over a year. I still wanted to go out and have fun, and I was socializing with people that were drinking and doing drugs. I felt like I had it nailed. I remember it was four in the morning, back at someone's house. And someone had cocaine and I just started to really, really want a line. I talked myself into doing it, thinking Oh it'll be fine, it's just one line."