Immune-mediated inflammatory diseases share several pathogenic pathways and this pushes sometimes to extrapolate from one disease or indication to others. A biosimilar can be defined as a biotherapeutic product which is similar in terms of quality, safety, and efficacy to an already licensed reference biotherapeutic product. We review the substrate for extrapolation, the current approval process for biosimilars and the pioneering studies on biosimilars performed in rheumatoid arthritis patients. A biosimilar has the same amino acid sequence as its innovator product. However, post-translational modifications can occur and the current analytical techniques do not allow the final structure. To test the efficacy in one indication, a homogeneous population should be chosen and immunogenicity features are essential in switching and interchangeability. CT-P13 (Remsima™; Inflectra™) is a biosimilar of reference infliximab (Remicade®). It meets most of the requirements for extrapolation. Nevertheless, in inflammatory bowel diseases (IBD) we need more studies to confirm the postulates of extrapolation from rheumatoid arthritis and ankylosing spondylitis to IBD. Furthermore, an effective pharmacovigilance schedule is mandatory to look for immunogenicity and side effects.