Despite the availability of effective treatment, over half of the patients being treated for hypertension drop out of care entirely within a year of diagnosis (15) and of those who remain under medical supervision only about 50% take at least 80% of their prescribed medications (16). Consequently, because of poor adherence to antihypertensive treatment, approximately 75% of patients with a diagnosis of hypertension do not achieve optimum blood-pressure control (13,18).

Estimates of the extent to which patients adhere to pharmacotherapy for hypertension vary between 50 and 70%. This variation relates to differences in study groups, duration of follow-up, methods of assessment of adherence and drug regimens used in different studies. For example, studies that defined adherence as an 80% ratio of days on which medication was dispensed to days in the study period, reported adherence rates ranging from 52 to 74% (19,20). Other studies that have investigated discontinuation of antihypertensives have reported adherence rates of 43 to 88% (21 - 24). Furthermore, it has been estimated that within the first year of treatment 16 to 50% of patients with hypertension discontinue their antihypertensive medications, and among those who continue their therapy in the long term, missed doses of medication are common (25). These figures differ for newly-diagnosed patients and those with chronic, long-standing hypertension (26).

Another source of variation that could explain the differences in rates of adherence is the method used to measure adherence. Examples of methods used include calculating the percentage of pills taken in a specific time period, the percentage of patients taking 80% of their pills, the improvement in number of pills taken, the dropouts from treatment and follow-up, and the missed appointments. There are also indirect proxy measures such as change in blood pressure and the achievement of target blood pressure (27).