Kira Yarmysh, a representative of Navalny, told Reuters an ambulance was called on April 7 to the Russian maximum-security IK-6 penal colony where he is being held.
Navalny, 46, was experiencing intense pain and could not eat the prison food as a result.
On Tuesday, Navalny’s social media account, run by his allies, tweeted that he was released from “shizo,” or a solitary confinement punishment cell, on the evening of April 7.
After a five-month recuperation in Germany, Navalny returned to Moscow in 2021, where he was taken into custody.
A documentary about Navalny won an Oscar last month.