Rayos is a proprietary delayed-release formulation of low-dose prednisone and is known as Lodotra outside the US.
Horizon Pharma chairman, president and chief executive officer Timothy Walbert said feedback from the initial Rayos launch has been positive.
"This gives us confidence moving forward as we have now implemented the full commercial launch of RAYOS through our 150-person sales force," Walbert added.
The company also announced the approval of Lodotra in South Korea and the approval triggered a milestone payment from its partner MundiPharma.