The singer has been battling back to full fitness since undergoing knee surgery in January (14) and he decided to team up with his personal trainer Jarvis for his first gym visit since the operation.
He was put through his paces at a fitness centre in Bogota, Colombia ahead of the opening night of the tour on Friday, but the rigorous workout, combined with the high-altitude atmosphere, sparked a nasty bout of vomiting.
Horan later posted a photo of himself on an exercise bike at the gym alongside the message, "First session back with Jarvis! Knee feelin (sic) ok! Threw up because of the altitude, me and Jarv called it #altitudeoosshhh (sic)."