Patients receiving a haematopoietic stem cell transplant (HSCT) are subjected to complex oral medications based on prophylactic and immunosuppressive treatments. Adherence to medication plays a role in survival, and medication non-adherence (MNA) is closely associated with graft-versus-host disease and other complications. The aim of this systematic review is to summarise the available evidence regarding prevalence rates of medication adherence, the risk factors of MNA, the effectiveness of interventions to increase medication adherence and the outcomes associated with MNA.