This is a proof-of-concept study in 30 patients with established Relapsing Remitting Multiple Sclerosis (RRMS). IMP is Famciclovir. It is a phase II type A open label study. Each individuals participation in the study will last 36 weeks and will be divided into three phases: pre-treatment (12 weeks), treatment with famciclovir (12 weeks), and post-treatment (12 weeks). During the first 12 week phase patients will remain on their usual treatment alone; this will be followed by three months of co-treatment with famciclovir and then followed by a final three months post-famciclovir treatment where participants will continue to take their usual treatment for RRMS. The primary aim is to explore the effect of famciclovir (500mg BD) on Epstein-Barr virus (EBV) shedding in the saliva of patients with MS

Inclusion Criteria:

• Diagnosed with MS
• Males and females aged over 18
• Taking Natalizumab (Tysabri) for the treatment of MS,
• Be able to provide informed consent to take part in this study.
• Female patients of childbearing potential (defined in section 13.4) must be willing to follow protocol contraceptive requirements.

Exclusion Criteria:

• Taking MS disease modifying treatment other than natalizumab (Tysabri)
• On no treatment for MS
• Taking additional immunomodulatory agents (either for MS treatment or other reasons)
• Has received a course of high dose oral or intravenous steroids within 3 months of study entry. The use of low dose (total daily dose <10mg) and/or the use of topical steroids will not act as an exclusion criteria.
• Taking antiviral or antiretroviral medication for any reason
• Known allergy to penciclovir, acyclovir, valaciclovir or famciclovir
• Taking probenecid
• Significant renal (CKD 3 or 4) and/or liver (ALT>3x ULN) impairment
• Pregnant, or unwilling to take measures to prevent pregnancy
• Breastfeeding