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.
“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 believe 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: Duncan Kennedy, BBC News
Parked outside the red brick Amesbury Baptist Centre are three police cars and four or five officers standing around with one jotting something down in a notepad.
There are no forensic tents, and it all feels like a very low key operation.
It doesn’t feel like a Hazchem or Hazardous Chemical incident. It’s not Skripal part two.
It does seem like an incredible coincidence for another incident like this to occur, it may just be the police being cautious or there could be something more to it.
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.
Amesbury: Two collapse near Russian spy poisoning site