Compliance with medical therapy in general is often low, and compliance with blood pressure treatment is no better. Numerous studies have shown that patients frequently drop out of treatment for hypertension. Furthermore, even when patients stay in treatment, they often take their medications in a way quite dissimilar from that prescribed. Identifying noncompliant patients is important but not always easy to accomplish. Pill counts, the "gold standard," are seldom practical in routine clinical practice. Assessing compliance by its biological effect is compromised by physiological diversity among patients. Assessing compliance from patient self-reports is limited in its accuracy but is more useful than many researchers and clinicians appreciate. Compliance behavior is affected by many factors. Complexity of medical regimen has some effect; the presence of drug side effects has surprisingly little. Contrary to what many clinicians think, increased age is often accompanied by increased medication compliance. Features of the doctor-patient relationship likely have an important effect on patient compliance, though our knowledge of these factors is still limited.
Lawrence E. Klein (1988) studied this question.