The American businessman jailed in Bolivia without charge was visited by two members of the US Congress last week.
Congress members Chris Smith and Nydia Velazquez sought to secure the release of grandfather Jacob Ostreicher, who has been in jail for 18 months and denies all allegations against him.
The 52-year-old grandfather has been accused of allowing his rice farming business to be used as a front for money-laundering for the drugs trade.
Mr Ostreicher, who has been held at Palmasola prison since June 2011, is currently on hunger strike.
Mr Smith, a New Jersey congressman, said on Friday: "Jacob’s health has steadily deteriorated during this unjustified captivity.
“The stress of his confinement and his hunger strike have taken their toll on his health since I saw him in June.”
Mr Ostreicher has claimed that corrupt officials have kept him in jail so they could sell 18,000 metric tons of rice that was confiscated in exchange for promises of release. Local authorities have subsequently arrested seven people, including officials, for extortion.
In response, Mr Smith said: “The arrests of a group of people connected to his ordeal have given us hope that recent progress can lead to his release.”