"I had to get off the plane to go to work, so I had to sleep on the plane," Renner, 41, recalled on Tuesday's Jimmy Kimmel Live! "So I take a little sleeping pill, pop it and realize nothing's happening – but something else was happening."
What he thought was Ambien was actually Viagra, given to him by a friend.
"Not only did I not sleep the entire flight," he added of his 10-to 12-hour trip, but he also had to deal with his pants "camping."
The flight crew, who were "sort of in on the joke," helped ensure an otherwise smooth voyage – with lots of ice.
Still, "There was no walking," he said. "If I had to go to the restroom, it was so embarrassing."