The 43-year-old, President Vladimir Putin’s most prominent critic, was rushed to hospital on Sunday with swollen eyelids, discharge in the eye and a rash on his upper body, doctor Anastasia Vasilyeva said.
“It is indeed poisoning by some unknown chemical substance,” his lawyer Olga Mikhailova told reporters outside Moscow’s hospital No 64, which treated Navalny.
Vasilyeva wrote on Facebook that “some toxic agent may be the reason for Alexei Navalny’s ‘illness’”.
Navalny has been in jail since last week for calling an unauthorised rally.
After his hospitalisation on Sunday doctors diagnosed Navalny with a skin condition, Vasilyeva said.
They treated him with a steroid and he got better, but he has been sent back to jail despite needing to continue the treatment, she told reporters.
She also said she suspected his prison cell could be contaminated.
Vasilyeva is an ophthalmologist who had treated Navalny previously for an eye injury.
She accused the hospital doctors of not wanting to find out what caused Navalny’s condition.
“Alexei has been clearly sent back to the detention centre on orders from above,” she said, referring to the authorities.
Navalny was jailed for calling a rally against authorities’ move to block prominent independent candidates from taking part in Moscow parliament elections in September.