The retired Ohio autoworker is being held in Germany on an arrest warrant accusing him of accessory to 29,000 murders as a guard at a Nazi death camp.
Demjanjuk was moved Sunday from Munich's Stadelheim prison to the nearby Harlaching clinic to be treated for the ailment, which is caused by an excess of uric acid in the blood, attorney Guenther Maull told The Associated Press.
"It was determined that his blood was not as it should be," Maull said. He said Demjanjuk was expected to return to the Stadelheim prison later Tuesday.
Demjanjuk's son, John Demjanjuk Jr., said in an e-mail that he had heard his father was suffering from low hemoglobin levels and "severe pains from other conditions" but that he has not been permitted to call him personally to check on him.
Meanwhile, Munich prosecutors have still not been able to determine whether the 89-year-old is healthy enough to stand trial, Maull said. He said he did not know specifically when a decision could come, and the prosecutors' office spokesman could not immediately be reached for comment.
Demjanjuk was deported from the U.S. earlier this month.
The Ukrainian native maintains he was a Red Army soldier who spent much of World War II as a Nazi POW and never worked in a camp.
But Nazi-era documents obtained by U.S. justice authorities and shared with German prosecutors include a photo ID identifying Demjanjuk as a guard at the Sobibor death camp and saying he was trained at the SS-run Trawniki camp.