West Midlands Police have warned of a "potentially lethal batch" of synthetic cannabis after a man was found dead near Birmingham's Bullring.
The homeless man, in his 30s, was believed to have taken a variety of the drug called Black Mamba. He was discovered on Sunday morning.
Two other people became ill in the city after taking the drug over the weekend.
Detective Inspector Thomas Hadley said: "It is extremely concerning that users of this substance are putting their lives at risk, even more so because we believe there is a potentially lethal batch of the substance currently being distributed in Birmingham.
"We will, of course, seek to help and support those who use these products, as we are very aware of the addictive nature of these substances and the serious harm they can cause, therefore we are keen to break the pattern of behaviour of those buying and using them.
"Those involved in the production and supply of these drugs will continue to face robust action to disrupt their activity as we seek to shut down the manufacture and supply chains across the region."
Synthetic cannabis – also known as "spice" – was recently reclassified as a Class B drug.
There have been growing concerns over its effects, particularly among young people and in prisons, where it has been linked to inmate violence.
Manchester also saw a large number of people falling ill because of the drug. Police said they had received 14 calls over the weekend about people who had collapsed after smoking the drug.