Fentanyl is a synthetic opioid that’s fifty times more powerful than heroin. It’s also the biggest killer of 18-45 year olds in the United States.

The death toll is so high that the sons of imprisoned drug lord El Chapo have said they aren’t going to sell it anymore - although experts say that’s probably just a publicity stunt.

Dr Smitha Mundasad from the BBC’s Health team explains why fentanyl is so fatal. The BBC’s Nadine Yousif in Canada talks about the fentanyl crisis’ ‘fourth wave’, which is affecting every community in the North America - including San Francisco, where activist Darren Stallcup is based.

Also in this episode, we talk about Argentina’s election results with the BBC’s Mimi Swaby.

Email: whatintheworld@bbc.co.uk WhatsApp: +44 0330 12 33 22 6 Presenter: Hannah Gelbart with Alex Rhodes Producers: Alex Rhodes and Benita Barden Editor: Verity Wilde and Simon Peeks