The recently poisoned couple were being treated at the same hospital that cared for the Skripals.

The hospital, in Salisbury, “has the most expertise in the world in dealing with people with this type of poisoning, but it very much depends on the state of the individual,” said Alastair Hay, an emeritus professor of toxicology at the University of Leeds, England. “The fact that someone is frailer makes it more difficult, makes the outlook a little more bleak.”

He said the couple’s prospects for survival were “much more problematic, I think, given their overall state.”

Scientific knowledge about Novichok poisoning was close to zero in March, when the Skripals and Sergeant Bailey fell ill, and medical staff members expected them to die, they told the BBC in May. A paper published in May in the scientific journal Clinical Toxicology noted that while a large dose of a nerve agent could kill a patient within minutes, “mild or moderately exposed individuals usually recover completely.”

Mr. Rowley’s symptoms were observed more closely than those of Ms. Sturgess or of the Skripals because a friend, Sam Hobson, was with him at the time. Mr. Hobson described symptoms that progressed over a period of hours, beginning with profuse sweating and fever, then hallucination and dribbling.

Each of the two recent victims had long histories of addiction.

Ms. Sturgess referred to her troubles with drinking in wry Facebook posts, writing in 2016 that, after one spree, she was not sure how she had reached her room at John Baker House, a supported-living facility in Salisbury that houses people with drug and alcohol problems. She thanked her mother for feeding her when she was sick.

“Love her, she knows how to mend me when I’m lost and low,” Ms. Sturgess wrote.

Her posts brightened in February 2017 when she began her relationship with Mr. Rowley, a recovering heroin addict who rented an apartment in Amesbury, eight miles away.

“Fell in love … never bodes well for me,” she wrote. “I trust Charlie with my life and he gets me the best gifts ever.”