Jared Lee Loughner's attorneys questioned whether the medication violates an earlier order by the 9th Circuit Court of Appeals that forbid them from involuntarily medicating Loughner as the court mulls an appeal on his behalf.
Officials at the federal prison facility apparently resumed the forced medication on an emergency basis because Loughner had become an immediate threat to himself.
Attorneys say their client has been on 24-hour suicide watch, but he denies having suicidal thoughts.
Loughner is accused of killing six people and wounding 13 others, including Congresswoman Gabrielle Giffords, in January.
The Associated Press contributed to this story.