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, both in their 40s, were found unconscious in Amesbury, Wiltshire, on Saturday.
Officers initially thought the collapse was drugs-related but have since said they are open-minded as to the cause.
Wiltshire Police said further tests on the substance were being carried out in a bid to identify it.
While it was not clear if a crime had been committed, the force said, a number of scenes 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, the force said.
They are both currently receiving treatment for suspected exposure to an unknown substance at Salisbury District Hospital, it added.
Public Health England (PHE) said it did not believe there was a significant health risk to the wider public, although its advice was being continually assessed.
Officials said the hospital was open as usual and advised patients to attend routine appointments unless contacted and advised to do otherwise.
Amesbury is about ten miles from Salisbury, where former Russian spy Sergei Skripal and daughter Yulia were poisoned by a suspected military nerve agent in March.
News Source BBCNews