The purpose of this clinical trial is to learn how well ACT-777991 works, how safe it is and how well it is tolerated by adults with non-segmental vitiligo.
The main question this clinical trial aims to answer is:
• Can ACT-777991 help return color to the skin of the face of adults with non-segmental vitiligo?
Researchers will compare ACT-777991 to placebo (a look-alike inactive treatment that contains no medicine) to see if ACT-777991 works to treat non-segmental vitiligo.
Trial participants will:
* Take the trial intervention (either ACT-777991 or placebo) daily for 24 weeks.
* Visit the clinic 7 times for check-up and tests.