Navalny accuses the prison authorities of using his health as a tool to put pressure on him.

Aleksei A. Navalny, the jailed Russian opposition leader who survived an assassination attempt that was believed to have been orchestrated by the Kremlin, says he is suffering from increasingly bad back pain after months in solitary confinement at a penal colony.

Mr. Navalny said in a series of posts on Twitter on Monday that he had been injected with unknown medication that failed to alleviate the pain, and he accused the authorities of deliberately withholding his medical records from him.

“If you lock a person up in a punishment cell, where he can either stand or sit on an iron stool for 16 hours a day, after a month in such conditions even a healthy person will undoubtedly get back pains,” Mr. Navalny wrote on Twitter. “I’ve spent the last 3 months like this. Naturally, my back hurts a lot.”

Mr. Navalny returned to Russia in January 2021 after recovering in a Berlin hospital from an assassination attempt and was promptly arrested. Weeks later, he was sentenced to two and half years in prison for violating the terms of his earlier parole while being in Germany. He was handed a new nine-year sentence in March, in the wake of Russia’s invasion of Ukraine, after prosecutors accused him of embezzling donations from his supporters.

Mr. Navalny became Russia’s most prominent opposition leader by exposing high-level corruption and by challenging President Vladimir V. Putin and his party, United Russia. White House officials have said that American intelligence agencies concluded that Russian security police agents had poisoned Mr. Navalny.

In his Twitter posts, Mr. Navalny said that the prison authorities were not allowed to use blunt physical force against him, but that they used other means to make him comply, including forcing him to share a cell with a dirty inmate.

This month, he was sentenced to 12 days in a punishment cell for using a swear word in a conversation with his cellmate, the ninth such sentence over the last six months, he said.

After repeated requests to receive medical aid, a doctor appeared, he said in a post. No diagnosis was disclosed to him, he said, and he was not sure what kind of medication he had been given.

Mr. Navalny said that weeks after requesting his medical records in an attempt to see his diagnosis and what he had been prescribed, he had finally received them. He attached what appear to be the records to his post, showing that they were copied in a way that rendered them almost unreadable.

Mr. Navalny has remained active while in prison at Russia’s notorious Penal Colony No. 2, regularly publishing updates about his time there and comments about the political and economic situation in Russia.

His political organization, which a few years ago had offices and activists in all major Russian cities, has been wiped out by the authorities. All of its top leaders have fled, and many activists have been arrested.