Mohammed Allan, 31, an Islamic Jihad activist, began the strike after his detention without trial in November. Barzilai hospital in Ashkelon said he had been put on a respirator and a saline drip and his condition was stable.
Israel has long been concerned that hunger strikes by Palestinians in its jails could end in deaths and trigger waves of protests in the occupied West Bank and East Jerusalem.
Last month Israel enacted a law permitting doctors to administer nutrition to hunger strikers against their will to try to keep them alive.
The law has met opposition from Israel’s medical association, which considers force-feeding a form of torture and medically risky and has urged Israeli physicians not to abide by it.
Allan was moved to Barzilai hospital from another hospital in southern Israel earlier this week and doctors at both hospitals have so far honored his wish not to be treated.
“The condition of Mohammed Allan deteriorated this morning. He is receiving treatment and his condition is stable. The treatment is being administered according the ethics committee guidelines and includes respiration and intravenous fluids and saline,” the hospital said in a statement.
It said that a doctor from the International Committee of the Red Cross had come to the hospital.
Some Palestinian administrative detainees, whom Israel has held without trial or pressing charges, have used hunger strike protests to try to effect their release. Israel has not outlined charges against Allan, who was detained in November.
In 2012, Israel freed Khader Adnan, another Islamic Jihad activist held without trial following a 66-day hunger strike. He was re-arrested and held for a year until his release last month.
In Gaza, the Islamic Jihad militant faction said it would hold Israel responsible if Allan died and hinted it could respond with violence.
“In view of (Israeli) intransigence and their rejection of the demands of Mohammed Allan we hold them fully responsible for his life and if he dies, it will force us to defend our people and our prisoners,” a statement said.
On Wednesday, some 200 supporters of Allan clashed with Israeli right-wingers near the hospital. Mounted police and other officers dispersed the fracas and a number of protesters were detained and questioned, a police spokeswoman said.