Arians -- who filled in for head Coach Chuck Pagano this season as the head coach battled leukemia -- was hospitalized but is "doing fine," the Colts said in a statement.
NFL.com reported the 60 year old was believed to be suffering from an adverse reaction to medication he had taken last week and was to spend the night hospitalized in Baltimore.
He had previously missed Thursday's practice because of flu-like symptoms.
Arians guided the Colts to a 9-3 record in Pagano's absence and is thought to be a candidate for several current head coaching vacancies around the league.
2014: The Year in Music [PHOTOS]