A lawyer for Nadia Savchenko says that the Ukrainian military pilot will be moved from a Moscow jail to a civilian hospital in the Russian capital on April 28.
Lawyer Mark Feigin said on Twitter on April 27 that he had just visited Savchenko in the Matrosskaya Tishina pretrial detention center.
"They will send her to City Hospital No. 20 tomorrow," Feigin wrote.
It was not immediately clear whether the transfer would be permanent.
A spokeswoman for the Russian Federal Penitentiary Service said hours earlier that Savchenko's health had deteriorated abruptly after she refused to eat over the weekend, and that she would be transferred to a civilian hospital "if needed."
Savchenko has been jailed since July, when she says she was illegally brought into Russia after being abducted by pro-Russian separatists in eastern Ukraine.
She is charged with complicity in the killing of two Russian journalists who died in the conflict there, as well as illegal border crossing.
She denies guilt and maintained a hunger strike for more than 80 days, ending in March, in protest against her incarceration in Russia.
In a handwritten letter posted on Twitter by Feigin, Savchenko wrote that she had an allergy of unknown origin and was being transferred to a civilian hospital for that reason.
Savchenko wrote that she weighs 50 kilograms and her body temperature is low.
Russian prosecutors announced the final charges against Savchenko on April 24 and said the investigation was complete, a sign that her trial could begin within weeks.
Kyiv and Western governments have repeatedly called for Savchenko's release.
Her jailing has added to already extreme tensions between Russia and Ukraine following Moscow's illegal annexation of Crimea in March 2014 and the eruption of the conflict in eastern Ukraine, which has killed more than 6,100 people.
Ukraine says she is subject to release under a February agreement on a cease-fire and other steps to end the conflict.