A court in the Democratic Republic of Congo last month found French guilty of strangling his cellmate and countryman Tjostolv Moland to death, despite evidence from the autopsy, which was overseen by Norway's criminal investigation service, indicating that this was not the case.
Hans Marius Graasvold, French's lawyer, told Dagbladet newspaper that French's condition had worsened noticeably since the trial.
"If he does not get help, then it's only a matter of time before he gets so bad that he can not go on anymore," he said.
Since the ruling, the Norwegian press whose presence had put pressure on the Congolese authorities to treat French humanely, have largely departed, leaving French's mother, Kari Hilde French, as his only regular visitor.
For two weeks after the ruling, officials from Norway's foreign ministry were also barred from visiting him in his cell.
French and Moland, who both formerly served in the Norwegian military, were arrested in 2009, accused of killing the driver they had hired as they attempted to set up business in the Congo.