Social affairs correspondent Robert Booth tells Madeleine Finlay why a class of synthetic opioids called nitazenes, first developed in the 1950s, is leading to a worrying number of fatal overdoses in the UK. And she hears from toxicology and addiction specialist Dr Joseph D’Orazio about a tranquilliser called xylazine that has been showing up in alarming volumes in the US illegal drug supply and is now starting to appear in toxicology reports in the UK