Police said investigators were baffled by the circumstance of their deaths.
Charlie Milson, a 35-year-old British, and Jonathan Louis, a 40-year-old American, were found sitting on chairs and their bodies were covered with some vomit. They had likely been dead for eight to 10 hours, police said.
There were no signs of struggle inside the room. Next to them were four bottles of drug rehabilitation medicine which had been recovered for testing.
A witness, Thamakorn Apison, told police that she had been living with Louis at a school inside a local shopping mall for 14 years and they moved to the condominium two years ago.
Ms Thamakorn said both men, who were friends, went into the room the night before. Next morning, she discovered that the door was locked so she asked a security guard to unlock it. It was then that they were found dead.
Police initially suspected that both foreigners probably had medical conditions. The British and US embassies will be contacted and medical examinations will be performed to determine the actual cause of death, police said.