Nadia Savchenko has lost at least 14 kilograms.
The Ukrainian Air Force pilot, who is being held in Russia on charges of complicity in the killing of two journalists, has already been given two glucose drips to prevent her from fainting.
Savchenko has been refusing food for more than a month, and her lawyer says the hunger strike has taken a toll on her health.
"It's deteriorating. She's losing weight with every passing day," Mark Feigin told RFE/RL. "She's become thin and hollow-cheeked. Changes are taking place in her body."
Savchenko maintains her innocence and rejects accusations that she tipped off Ukrainian forces about the location of Igor Kornelyuk and Anton Voloshin, two Russian journalists who were killed in June when they were hit by mortar fire while reporting in eastern Ukraine.
In an open letter released this week, she vowed to starve to death if Russia does not release her.
Knowing her ironclad resolve, lawyers are taking her pledge seriously.
"It's hard to break her," says Feigin. "We discuss this matter every day with her, but Nadiya believes that ending her hunger strike would amount to weakness."
Feigin says authorities at her pretrial detention center are also getting nervous and have urged Savchenko to end her hunger strike.
She is a high-profile inmate, and any harm she sustains under their watch could incur the wrath of both the Kremlin and the international community.
"They visit her every day," he says. "The guards watch her closely because she could faint, fall, and hit her head."
Batkivschyna Party leader Yulia Tymoshenko (center) reads an appeal for international support for the release of Nadia Savchenko during a parliament session in Kyiv on January 13.
In Ukraine, too, many are entreating Savchenko to call off her hunger strike, including former Prime Minister Yulia Tymoshenko; parliament speaker Volodymyr Hroysman; the head of the Orthodox Church of Ukraine, Metropolitan Onufri; as well as the pilot's family and friends.
Feigin himself believes Savchenko must live to either be released or see her trial out.
"From experience, I think this is not an effective measure," he says. "True, she is drawing a lot of attention to her case, but this won't influence the prosecution. Russian authorities are deaf to such things. If she dies, she dies."
Ukraine's parliament has chosen Savchenko, who is also a lawmaker, to represent the legislature at the Parliamentary Assembly of the Council of Europe (PACE).
But Feigin has doubts the move, aimed at pressuring Russia into releasing Savchenko in time for PACE's next session on January 26, will succeed.
There's a possibility, he says, that Russian authorities will draw out the trial in a bid to erode public interest in the case.
The prejudicial inquiry into Savchenko was opened more than six months ago. Under Russian law, it can last up to two years.
"They still have time," says Feigin.
The lawyer, however, says Moscow risks a massive international backlash if Savchenko dies or is sentenced to jail time.
"This is not an ordinary political case. This is about war, about the death of people," he says. "If Russian authorities try to convict her despite the evidence of her innocence, this will not be a quiet trial for Russia. The Kremlin is already suffering serious consequences by pursuing this case, and these consequences will increase dramatically."