Dr. Karl Max Einhäupl, the head of the Charité, said that the doctors who had cared for him in Moscow had “done a good job” and been “very cooperative” in sharing information with their colleagues in Berlin.

Mr. Verzilov’s mother, Elena Verzolova, and Ms. Nikulshina said they believed he might have been deliberately poisoned by the Russian security authorities. They decided to take him out of Russia, and accompanied him to Berlin.

“I think he will be more safe here,” Ms. Verzolova said, adding that she felt reassured to have a second opinion on his condition.

“It would be really dangerous for him to go back, because probably it was assassination attempt — if not it was intimidation — because probably he knew something that he wasn’t supposed to know,” said Nadezhda Tolokonnikova, one of the original Pussy Riot activists, who used to be married to Mr. Verzilov and met the family in Berlin.

Pussy Riot, widely known as a punk band unsparing in its criticism of President Vladimir V. Putin and the Russian government, gained notoriety in 2012 when three of its members were sentenced to two years in prison on charges of hooliganism, leading to worldwide protests.

Mr. Verzilov and Ms. Nikulshina were two of the four activists who interrupted the soccer World Cup final on July 15 that was attended by Mr. Putin. They were sentenced to 15 days in jail each.

Two weeks ago, Ms. Nikulshina was once again arrested at a traffic stop and was detained by the police for two nights, she said. On the day he fell ill, Mr. Verzilov and other friends had spent the afternoon in and around the courthouse, waiting for her release.