Nonadherence to medicines is a global problem compromising health and economic outcomes for individuals and society. This article outlines how adherence is defined and measured, and examines the impact, prevalence and determinants of nonadherence. It also discusses how a psychosocial perspective can inform the development of interventions to optimise adherence and presents a series of recommendations for future research to overcome common limitations associated with the medication nonadherence literature. Nonadherence is best understood in terms of the interactions between an individual and a specific disease/treatment, within a social and environmental context. Adherence is a product of motivation and ability. Motivation comprises conscious decision-making processes but also from more 'instinctive', intuitive and habitual processes. Ability comprises the physical and psychological skills needed to adhere. Both motivation and ability are influenced by environmental and social factors which influence the opportunity to adhere as well as triggers or cues to actions which may be internal (e.g. experiencing symptoms) or external (e.g. receiving a reminder). Systematic reviews of adherence interventions show that effective solutions are elusive, partly because few have a strong theoretical basis. Adherence support targeted at the level of individuals will be more effective if it is tailored to address the specific perceptions (e.g. beliefs about illness and treatment) and practicalities (e.g. capability and resources) influencing individuals' motivation and ability to adhere
