Sen. Tim Johnson (D-S.D.) was admitted to a hospital Monday with a fever.

According to his office, Johnson was admitted to the Avera McKennan Hospital & University Health Center in Sioux Falls, S.D., after coming down with a fever believed to be related to an allergic reaction to an antibiotic.

Dr. Scott Beckstrand said Johnson was in stable condition. “We are doing a full work-up to determine the cause, but we suspect the cause could be a medical drug reaction,” he added.

“He’s in good hands and good spirits,” a Johnson spokeswoman said.

Johnson, 63, suffered a stroke related to an arteriovenous malformation in 2006.