Pyotr Verzilov, a member of Russian protest group Pussy Riot, showed "highly plausible" symptoms of poisoning, German doctors treating the dissident said on Tuesday.
Dr Kai-Uwe Eckardt of Berlin's Charite hospital said Verzilov received intensive care since arriving in Berlin from Moscow on Saturday, but his condition was not life-threatening.
Mr Verzilov's symptoms and information received from relatives and the Moscow hospital he was admitted to last week "indeed make it highly plausible that a poisoning took place," Dr Eckardt said.
Charite doctors found "no evidence whatsoever that there would be another explanation for his condition," he also said.
Dr Eckardt said Mr Verzilov was treated well in Moscow before being flown to Germany. He says the symptoms indicate Mr Verzilov is suffering from an anticholinergic syndrome. This can come from disruption of the nervous system which regulates the internal organs.
Although doctors in Berlin have not yet determined what caused the poisoning, they said it could be various substances, including high doses of some pharmaceuticals and plants which contain particular toxins.
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Dr Karl Max Einhaeupl, the Charite hospital's chairman, said doctors wanted to "refrain completely from all speculation about what made these problems happen".
Although he wouldn't rule out that recreational drugs could be responsible for the poisoning, he said such drug use is rare.
"We have no evidence that there is a drug problem and it would be very unusual for someone to take a drug in the dose that it was taken," he said. "That would be done with suicidal intent, but we have no indications of this."
Mr Verzilov and other members of the Pussy Riot group served 15-day jail sentences for disrupting July's World Cup final to protest what it said were human rights abuses in Russia.
