In Mexico, poppy farming and the production of opium paste – heroin's main ingredient – have long been the main source of revenue for many impoverished rural communities. But this is no longer the case, with both organised crime and addicts now turning to fentanyl – a cheap, ultraconcentrated and synthetic opioid – as their drug of choice. Our Mexico correspondents Laurence Cuvillier and Matthieu Comin report from Guerrero State, where a rural exodus is underway.