Nadezhda Tolokonnikova was admitted to the hospital at the prison where she is serving a two-year term for a protest against President Vladimir Putin in Russia's main Orthodox Christian cathedral.
Her husband Pyotr Verzilov said the acting head of the prison had described his wife's condition as "horrible" but had given no further details.
He also said prison officials would not show him documents about her transfer to hospital or allow her lawyers to visit, Reuters reports.
The administration of the prison could not immediately be reached for comment, and prison service employees in the remote Mordovia region declined to comment.
The 23-year-old announced on Monday that she was starting a hunger strike to protest against "slave labour" at the prison camp where she is serving her sentence.
She said inmates were forced to work up to 17 hours a day, deprived of sleep and subjected to collective punishment and violence from senior inmates enforcing order.
(Via Radio NZ News)