Abdullah Elshamy was arrested on August 14 when police dispersed supporters of ousted Islamist president Mohamed Morsi in Cairo, which led to clashes in which hundreds were killed.
Elshamy, who works for the Qatar-based network, went on hunger strike in January to protest his detention and has yet to face trial.
He is suffering from "anaemia, the start of kidney failure, low blood pressure and hypoglycemia," his brother Mosaab said.
At the start of his hunger strike Elshamy drank juice and other sugary drinks but has been taking only water for the past month, his brother said.
And he has shed around 40 kilogrammes (88 pounds) over the past 112 days, he added.
"He can go into a coma if he doesn't take perfusions (drips) and if his blood sugar remains low... It is the start of the most dangerous period," he added.
Elshamy's lawyer, Shaaban Saeed, said that he was accused of joining a "terrorist group" and spreading false news.
"My client is paying the price for working for a channel that opposes the ruling regime," Saeed said.
The military-installed authorities have been incensed by Al-Jazeera's coverage of their crackdown on Morsi's supporters since July when the army ousted him.
On May 3, a court remanded Elshamy into custody for another 45 days, and Elshamy appeared in court looking gaunt.
His wife Jihad Khaled, meanwhile, was also on hunger strike, her mother Houda Abdelmoneim told AFP.
"Jihad began her hunger strike from March 14 to express solidarity with her husband," Abdelmoneim said, adding that her daughter fell unconscious yesterday and was taken to a hospital as she too had been taking "only water."
Three other Al-Jazeera journalists, who work for the network's English-language channel, are held in Egypt and on trial for defamation and supporting Morsi's blacklisted Muslim Brotherhood.
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