Alexei Navalny, Russia’s most prominent opposition politician, is dealing with a severe stomach ache in jail that could be some kind of slow-acting poison, his spokesman said on Thursday.

An ambulance was called for Navalny overnight from Friday to Saturday to the IK-6 maximum-security penal colony in Melekhovo, about 115 miles east of Moscow, where he is being held. Kira Yarmysh, his spokeswoman, told Reuters.

Navalny suffered from severe stomach pain, he said, and could not eat the food he was given in prison because it made his pain worse, and since Monday he has been prohibited from buying alternative foods.

“He doesn’t eat anything because he is prohibited from receiving food parcels or buying food from the prison shop and the food provided by the prison actually makes his stomach ache worse,” Yarmysh said in English.

“His health is not good,” he said. «We cannot dismiss the idea that they are poisoning him, not in large doses as before, but in small doses so that he does not die immediately but rather suffers and ruins his health.»

Yarmysh said there was no definitive proof of the poisoning theory, but he had never had such stomach pains before. She said that she was terrified for him because she had almost no contact with him and she was not receiving proper medical attention.

When asked about claims that Navalny might be slowly being poisoned, the Kremlin said it was not following the state of his health and that it was a matter of the federal prison service.

The prison service, which has in the past denied allegations that its employees mistreated Navalny and said he always received medical treatment when he needed it, did not immediately respond to a Reuters request for comment.

Alexey Navalny during a hearing against him in Pokrov, Russia, in February 2022. Ilya Pitalev/Sputnik via AP

Navalny, who is serving combined sentences of 11 1/2 years for fraud and contempt of court on charges he says were fabricated to silence him, said via Twitter on Tuesday that he had been moved back to solitary confinement and forced to endure conditions » extremely hellish.»

Yarmysh said he had suffered a similar stomach ache in January after being treated with antibiotics for a virus and had lost a lot of weight again.

The German government said on Wednesday that it was very concerned about Navalny’s worsening state of health. Reuters was unable to independently verify Navalny’s health status.

Navalny is a former lawyer who rose to fame more than a decade ago satirizing President Vladimir Putin’s elite and voicing allegations of large-scale corruption.

Russian authorities see him and his followers as extremists with CIA ties trying to destabilize Russia. They have outlawed his movement, forcing many of his followers to flee abroad.

In 2020, Navalny survived an apparent attempt to poison him during a flight in Siberia, leading Western laboratory tests to determine it was a nerve agent. Navalny accused the Russian state of trying to kill him, something he denied.

He was treated for that poisoning in Germany but voluntarily returned to Russia in 2021, where he was arrested upon arrival and imprisoned.

Yarmysh said that the medicines sent to the Navalny prison by his mother were not picked up by prison officials from the post office and were returned.

His supporters had to fight with prison authorities every time he fell ill to ensure he received some form of treatment, he said.

“Abusing Alexei’s health is standard practice in colony (prison) number six. All we can do now (to help him) is to talk about Alexei everywhere,» Yarmysh said.