Mohammed Allan, an alleged activist for the Islamic Jihad militant group, was on a respirator and was being given fluids, an Israeli hospital said.
His case is being seen as a test of a recent Israeli law authorising the force-feeding of hunger strikers.
Israeli doctors are challenging the law, which they say is unethical.
The Israeli Medical Association has described force-feeding as tantamount to torture and a violation of a patient's rights. It has urged doctors to refuse orders to perform the procedure.
Supporters of the law say force-feeding is necessary to save prisoners from death, and to discourage them from using hunger strikes as a tactic to end - or draw attention - to their detention.
It is not yet known if Mr Allan will be subjected to the controversial procedure, in which doctors forcibly insert a tube into the empty stomach. The decision could depend on his medical condition.
A statement from the Barzilai hospital, where Mr Allan is being treated, said his condition had deteriorated on Friday morning but remained stable.
"The treatment is being administered according the ethics committee guidelines and includes respiration and intravenous fluids and saline," a hospital statement said.
Jamil al-Khatib, a lawyer for Mr Allan, told the AFP news agency that he had heard his client had gone into a coma and that Israeli authorities were planning to force-feed him.
However, an Arab-Israeli MP, Osama Saadi, told the Associated Press news agency that Mr Allan was not being force-fed and was not in coma.
Mr Allan has been held without charge since November.

The SEP V fund will invest up to £20m in growth-stage technology businesses based mainly in the UK and Ireland.
It will also play "an active and supportive role" in their development.
SEP said investments in companies in other European countries would also be considered.
According to SEP, the fund was over-subscribed and closed above target.
SEP managing partner Calum Paterson said: "This is another great milestone for us and reflects extremely well on the calibre of our team.
"The new fund gives us a very strong platform to continue to invest in companies with world-class potential and we thank all of our investors for their support."
SEP portfolio companies currently employ more than 5,500 people and have aggregate revenues of more than £1bn.
They include Edinburgh-based travel search company Skyscanner and online car finance specialist Zuto, which is based in Manchester.
Non-UK companies in the portfolio include Berlin-based language learning company Babbel and Dublin-based e-commerce analytics company Clavis Insight.
SEP has 45 partners and employees across its Glasgow, London and Edinburgh offices.

8 March 2016 Last updated at 10:42 GMT
Peter Slevin, her biographer, talks about her legacy.
He looks at the first lady to do a hula hoop on the White House lawn and dance in public to Uptown Funk.
Mrs Obama has also taken a proactive stance on education and obesity among young people.

Glencoe, Loch Ness and Loch Lomond have been ranked among the top 10 best views in the UK.
The highest place went to Snowdon in Wales, followed by the Three Sisters at Glencoe.
Stonehenge in Wiltshire came out third, but we asked for your favourite views from around Scotland.
Here we've collected some of the very best of the pictures you've sent us.
A Palestinian held without charge by Israel has lost consciousness after nearly two months on hunger strike in protest at his detention.
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Glasgow-based venture capital firm Scottish Equity Partners (SEP) has raised £260m for a new fund that will focus on high-growth technology companies.
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The White House's first black first lady Michelle Obama once told her aides not to "just put me on a plane, send me someplace and have me smile".
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