Navalny, one of the country's most renowned critics of President Vladimir Putin, was admitted to hospital from jail, where he was serving a 30-day sentence for inciting protests, on Sunday.
In a Facebook post, Anastasia Vasilieva, an ophthalmologist who treated Navalny in 2017 after a pro-Kremlin activist threw green dye in his face, leaving him temporarily blind in one eye, said the circumstances of his most recent hospitalization led her to believe that a "toxic agent" could have been involved.
Vasilieva said she and Navalny's other doctor, Yaroslave Ashikhmin, had not been given proper access to the patient and that routine tests for cases of fast onset allergic reactions had not been undertaken, Efe news reported.
She added that information about Navalny's condition had not been made readily available to his family and friends, but officials had addressed the matter with the news agency Interfax.
The doctor said Navalny had no history of allergies and that his symptoms corresponded with dermatitis and acute keratoconjunctivitis.
In an earlier post, she denounced that one of the doctors at the hospital was a member of Moscow's legislative assembly for Putin's United Russia party.
Russian authorities detained Navalny on July 24 after he called for unauthorized anti-government protests in the capital against allegedly rigged city council elections.
On Saturday, over 1,000 people were arrested for taking part in the demonstration - protests in Russia must be authorized by the authorities and are tightly controlled.
Protesters showed up to demand that Moscow officials lift the ban on independent candidates running in the city elections.
Navalny, a lawyer, has been in and out of jail for inciting protests and is one of the country's most visible Putin detractors.
Russia's electoral commission banned him from running against Putin in the 2018 presidential elections citing an embezzlement conviction that Navalny has insisted is purely political.
( With inputs from IANS )