Oxytocin is the first-line treatment for uterine atony because it directly stimulates uterine smooth muscle contraction, promoting effective uterine tone to reduce bleeding. It has a rapid onset of action and is highly effective in preventing and managing postpartum hemorrhage caused by uterine atony. Oxytocin is preferred due to its safety profile and ease of administration compared to other uterotonic agents. Early administration is critical to minimize blood loss and prevent complications. Other agents like methylergometrine or misoprostol are reserved for cases where oxytocin is insufficient or contraindicated.

Reference: Williams Obstetrics, Cunningham FG, 25th Edition.