Fentanyl is deadly. Thousands of Americans die every year from a drug overdose – the majority of them after using a synthetic opioid like fentanyl. It was developed as a legal, and effective, pain killer. Now, fuelled by insatiable US demand, it is illicitly produced in makeshift laboratories in Mexico by organised crime groups. In the first part, Assignment travels to the Mexican Pacific port of Manzanillo. This is one of the main entry points for the chemical ingredients required to make fentanyl. It is a town where Mexico’s powerful cartels have fought for control, and where the mayor lives under armed guard after a failed assassination attempt. In the second part, we cross the border into the US from Mexico to explore the devastation this lethal drug has left in its wake in San Diego County. Presenter / producer: Linda Pressly Producer: Tim Mansel Producer in Mexico: Ulises Escamilla