Police sealed off a street in south Manchester after a man consumed an "unknown substance" believed to be ricin.
Photographs posted online show firefighters wearing hazmat suits at the scene in Northern Moor, Wythenshawe, on Saturday.
Greater Manchester Police (GMP) said it was called to the incident shortly before 9am following concerns about a man's welfare at a property on Moor Lane.
"It was established that the man had consumed an, at this time, unknown substance," a GMP spokesman said.
The man, who is in his 20s, was initially treated at the scene and remains in a stable condition, police said.
It is understood he consumed poisonous seeds.
The North West Ambulance Service tweeted that it had taken one person to hospital.
GMP said there is "no wider threat" to the public.
A witness said: "His mother came out shouting in her pyjamas 'Where are they? My f****** son is dying in there. Where the f*** is the ambulance.
"The fire service entered first with those bio and hazmat suits on and pushed a gurney into the house."
A cordon was put in place around Moor Lane, which was closed to traffic.
