A pill to treat multiple sclerosis that could cost a patient a whopping $54,000 a year is to be added to the PBS from January 1.

Scott Morrison is expected to announce on Sunday the addition of the pill called Mavenclad to the Pharmaceutical Benefits Scheme, News Corp Australia reports.

The plight of MS patients is particularly close to the prime minister's heart as his brother-in-law Garry Warren is a sufferer. With the PBS listing, patients will pay just $40.30 a script or $6.50 if they are concessional patients.