The Voice of the Malaysian People rights group (Suaram) told journalists the detainees had started a hunger strike on Tuesday to press for their freedom.
Two of the detainees who began the strike earlier have already been admitted to the Taiping hospital.
The 43 prisoners, some of whom are alleged members of the Jemaah Islamiya (JI) Islamist network, are being held at the Kemunting detention centre in northern Perak state, under the Internal Security Act (ISA).
One of those detained, Nik Adli, is the son of the spiritual leader of the opposition Islamic Party (PAS) - Nik Aziz Nik Mat.
Hunger strikers' details
Suaram spokesman Yap Swee Seng said the two sick hunger strikers were Muhammad Rashid Ismail and Asfawani Abd Allah - both detained in 2001 without trial or hearing.
Twenty-seven are alleged members of the JI, which is blamed for the Bali bombings that killed 202 people in October 2002.
The other 16 are accused of belonging to the Malaysian Militant Group (KMM), which is alleged to have international Islamist links as well.
All deny the allegations and ask for release or an opportunity to defend themselves in court.
They have all written to Prime Minister Abd Allah Ahmad Badawi to take up their cause.
A total of more than 80 alleged Islamists are held under the ISA, which allows for two-year detention periods that can be renewed indefinitely.
Rights groups have urged the government to scrap the legislation but the government maintains that detention without trial is needed as "a first line of defence against terrorism".