The word 'adherence' refers to the provision of consistent support (eg. for a political party or religion) or the act of holding particular elements together (eg. in constructing a building). In the medical context, adherence refers to the extent to which patients take their medications as prescribed. While the terminology related to medical adherence has changed significantly over the past two millennia, the core issue has not. Most recently, the term 'adherence' has replaced the term 'compliance', although it still jostles with 'concordance' in a growing literature which focuses now, as always, on one key question: why do so many people seek treatment for illness but then decide not to take their prescribed medication? This is an important question, both in terms of public health and societal cost: in the US, up to 50% of patients do not take their prescribed medications, resulting in additional healthcare costs of $290 billion per year. The greatest cost of non-adherence, however, relates to prolonged illness, increased rates of relapse and reduced wellness.