More than 100 inmates at the Kantharalak prison in Si Sa Ket have suffered weakness, dizziness and vomiting, and two have died.
Officials suspected vitamin deficiency and contaminated food and water were to blame.
Health officials started to examine about 800 inmates at the prison on Tuesday after two prisoners died in their cells and more than 100 others showed symptoms.
Narat Sawetanant, director-general of the Corrections Department, said on Thursday that health officials suspected vitamin B1 deficiency among the inmates, and were giving them injections of the vitamin.
Health officials also collected samples of chicken meat earlier served to inmates, together with samples of drinking water purified from groundwater, and planned to run tests to find out if the sickness resulted from contamination, he said.
He expected results in a few days.
A 54-year-old inmate died at the prison on Tuesday, and a 25-year-old on Dec 7, Pol Col Narat said.
About 10 inmates were sent to hospitals for treatment and two remained hospitalised on Thursday.
