A man and woman feared to have been exposed to an unknown substance are in a critical condition, prompting police to declare a major incident.
The pair, in their 40s, were found unconscious in what was thought to be in a drugs-related incident in Amesbury, Wiltshire, on Saturday.
Further tests on the substance are being carried out to identify it.
Amesbury is about 10 miles from Salisbury, where a former Russian spy and his daughter were poisoned.
Sergei Skripal and daughter Yulia were poisoned with Novichok, a suspected military nerve agent, in March.
Police said they are “open-minded” about the cause after a couple were found at a house in Muggleton Road, Amesbury.
While it was not clear whether a crime had been committed, the force said, a number of places in Amesbury and Salisbury known to have been frequented by the couple had been cordoned off.
- Russian spy: What happened to the Skripals?
“It was initially believed that the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of drugs,” Wiltshire Police said.
“They are both currently receiving treatment for suspected exposure to an unknown substance at Salisbury District Hospital,” it added.
The couple are believed to have attended a family fun day at Amesbury Baptist Church on Saturday afternoon before they were found unconscious in the property.
Roy Collins, church secretary, said “nobody else has suffered any ill-effects” from attending the community event and “there was nothing going on that was nefarious”.
“There have been no reports of any other incidents,” he said.
“We are all quite puzzled and shocked – naturally the connection with Salisbury and recent events there mean there is a heightened public interest.”
At the scene: Charlotte Callen, Home Affairs Correspondent
Neighbours tell me the peace at this usually quiet new estate in Amesbury was broken at about 06:30 BST on Saturday evening.
It was hot and many were out having barbecues when they heard sirens and saw flashing lights as first ambulances, then the fire brigade and police arrived in Muggleton Road.
They saw seven fire engines and fire officers wearing hazmat suits at the scene. The house was cordoned off and the word here was that this was a suspected drugs overdose.
This morning many are in disbelief that this has been declared a major incident and that so many reporters from national media have descended on their doorsteps.
Two police cars and three officers are here as Wiltshire Police is yet again dealing with a major incident just a few miles away from where Sergei and Yulia Skripal were found.
Wiltshire and Dorset Fire Rescue Service said seven vehicles and two specialist ones had attended Muggleton Road on Saturday evening.
Crews in hazmat suits, which are worn to deal with dangerous or hazardous substances, were sent to the scene in what the fire service described as a “standard procedure response”.
Chloe Edwards, a 17-year-old college student who lives near the cordoned-off flat, said she was having dinner when “all these emergency vehicles turned up” and was told to stay inside.
“They were putting on these green suits and we thought it was the gas as our electricity was turned off as well,” she said.
“We wanted to know what happened and with the Russian attack happening not long ago we just assumed the worst.
“I am not sure how long they were in there but we couldn’t get out of the house for two-and-a-half hours.”
Local resident Jake Murphy said he was watching the football on Tuesday night when “all these police cars appeared”.
“I didn’t know what was happening,” he said.
Justin Doughty, who lives opposite the police cordon, said none of the residents have been “told anything by the police”.
“We would have thought the police would have been more forthcoming and keep us in the loop.”
A government spokesman said ministers were “being kept up to date about the incident in Amesbury”.
Wiltshire’s Police and Crime Commissioner Angus Macpherson said the police had worked hard at “containing any risk that might be there”.
“There’s no reason to think it’s connected with matters of last month,” he said.
“I haven’t seen anything in this incident yet that I would consider to be an overreaction in terms of previous incidents, it all seems fairly textbook.”
Public Health England said it did not believe there was a “significant health risk” to the wider public, although its advice was being continually assessed.
The hospital was “open as usual” and advised patients to attend routine appointments unless contacted and advised to do otherwise.