Obesity is a possible risk factor of multiple sclerosis (MS), but the association between obesity and MS disease activity has not been explored. In a cohort of 86 MS patients, 80% of overweight or obese patients (BMI≥25kg/m(2)) had MRI activity compared to 48% of the normal-weight patients (BMI<25kg/m(2)) (p=0.001) during interferon-beta treatment. NEDA-status (no evidence of disease activity) was defined as a composite that consisted of absence of any relapses, sustained disability-progression and MRI-activity. Among normal-weight patients 26% obtained NEDA-status compared to only 13% of patients with BMI >25 (p=0.05). This may indicate that BMI affects interferon-beta treatment response.
Kvistad SS, Myhr KM, Holmøy T, Šaltytė Benth J, Wergeland S, Beiske AG, Bjerve KS, Hovdal H, Lilleås F, Midgard R, Pedersen T, Bakke SJ, Michelsen AE, Aukrust P, Ueland T, Sagen JV, Torkildsen Ø. Body mass index influence interferon-beta treatment response in multiple sclerosis. J Neuroimmunol. 2015: 288:92-7.
You can read the conclusions. The amount of drug per kilograme is effectively reduced if you are taking a standard dose and you are overweight so you may end up getting a relatively lower dose that may have reduced efficacy.