The jailed Kremlin critic has lost several kilograms since starting a hunger strike at the end of March. Navalny's allies say he has been refused access to his personal doctor.
Alexei Navalny's aides said Monday that prison officials are "threatening to force-feed him."
The prominent critic of Russian President Vladimir Putin began a hunger strike last month. He said that authorities at the penal colony where he is being held had refused him proper treatment for acute back pain.
According to his team, Navalny now weighs just 77 kilograms (169 pounds), a drop of 15 kilograms since he arrived in prison in February.
"Seeing the seriousness of the hunger strike, the administration is threatening every day to start force-feeding," they posted on his Twitter account.
They also said Navalny, who last week reported having a cough and fever, had been transferred back to the prison barracks from its infirmary.
However, prison administrators "are still not allowing an outside doctor to see him."
Allies of the jailed dissident use Twitter to provide updates based on information received from his lawyers who regularly visit him.
Navalny's lawyers and team use social media to share news of his status in prison. This screenshot of an Instagram post was published on March 15.
Russian officials counter that they have offered him proper treatment, but that he has refused it. They insist that he wants to be treated by a doctor of his choice from outside the facility, a request they have declined.
The Kremlin has insisted Navalny's celebrity does not entitle him to any special treatment in prison.
Navalny is imprisoned in a penal colony in the town of Pokrov, 100 kilometres (60 miles) east of Moscow,
An outside view of Penal Colony No 2 where opposition leader Alexei Navalny is serving his sentence.
Navalny is Russian President Vladimir Putin's best-known opponent. Last August he barely survived being poisoned with the nerve agent Novichok.
He was later arrested in mid-January after returning to Russia from Germany, where he had been treated for the poisoning.
A Russian judge sentenced Navalny to two and a half years in prison on old embezzlement charges.
mb/rt (AFP, Reuters)