Natalizumab is an effective drug in the treatment for relapsing remitting multiple sclerosis (RRMS) and is approved by de FDA/EMA in a treatment regimen of 4-weekly 300mg natalizumab infusions. Natalizumab trough concentrations after a 4-weekly interval are high in the large majority of patients which implies a relative overdose in most patients. A recent randomized controlled trial (RCT) suggests natalizumab maintains a high level of efficacy in stable patients with RRMS switching to a 6 week interval. Our study group demonstrated that efficacy of natalizumab is maintained when the infusion interval is extended based on natalizumab trough concentrations (personalized extended interval dosing). This leads to fewer hospital visits, a decrease of healthcare costs and decrease of risk of complications of natalizumab treatment.