United Kingdom

Evidence suggests dozens of suppliers are advertising openly on the internet and sending nitazenes in the post from China, where they are manufactured in laboratories. The majority of suppliers claimed to work for companies that otherwise appeared legitimate, with professional websites and business addresses in Chinese cities.

Nitazenes, which are illegal in the UK, are synthetic drugs produced in laboratories. They are similar to heroin and morphine, but can be several hundred times more potent.

It's thought users often take them unknowingly - because they are hidden within other illegal substances by dealers looking to cut production costs.

Nitazenes have been found by a publicly funded testing lab in a range of drugs, including street heroin and black market pills which dealers had wrongly claimed contained anti-anxiety drugs, such as Xanax and Valium.

After the BBC alerted SoundCloud, it removed the posts. X, formerly Twitter, took down hundreds but many listings remain.