This study aims to assess the real-life safety of Lenalidomide/Dexamethasone (Len/Dex) in patients with (Relapsed or Refractory Multiple Myeloma (rrMM), by evaluation of adverse events of special interest (infections, gastrointestinal events, thrombocytopenia, neutropenia, febrile neutropenia, and thromboembolism). In addition, due to the scarce information regarding the impact of frailty in Len/Dex treatment in rrMM, this study will prospectively assess Len/Dex safety and effectiveness by patient frailty groups.