Despite the recent emphasis on 'concordance', many people still do not take their medicines as prescribed. The extent to which this causes harm depends on the context of 'noncompliant' behaviour. The Medicines Partnership commissioned a review of literature published between 1996 and 2002 on noncompliance with treatment in: arthritis; cancer; coronary heart disease; depression; diabetes; epilepsy; hormone replacement therapy; osteoporosis; Parkinson's Disease; renal disease; and post-transplantation care. Electronic databases, peer reviewed journals, condition-specific journals and websites were searched. In some instances specialists were contacted.

The review revealed varying levels of noncompliance. There is evidence that only one third of patients continue to take at least 90% of lipid lowering treatment after two years. A review of research on noncompliance with immunosuppressive therapy in post-transplant patients reported rates up to 68%. Factors associated with poor compliance included complex regimens, experience of adverse side effects, patients' belief-related concerns about medication, cognitive or physical impairment, and illness denial.

Adverse effects of noncompliance may in some cases be life threatening. Nine out of ten late acute heart transplant rejections appear to be caused by noncompliance. Noncompliant cancer patients may have shorter relapse-free and total survival times, although the significance of observations such as this require critical analysis.

Individuals are less likely to take medicines seen as having a preventative function than those prescribed for curative reasons and symptomatic relief. For example, a quarter of women dropped out from a clinical trial investigating the cancer drug tamoxifen as a chemopreventative, compared to 17% of women who were taking it as an adjuvant therapy. Such observations have important implications in areas such as cardiovascular disease.

Interventions found to be successful in improving compliance include those which enhance self efficacy and confidence in medicine taking, those which combine educational and behavioural reward components with emotional and peer support, and medication reviews by pharmacists. However, additional research by the authors indicate that the role and significance of interventions such as those embodied in the Expert Patient Programme is often misunderstood, not least by health professionals.

Noncompliance is found in both the young and elderly populations. Around 40% of children and adolescents with renal disease are noncompliant with their medication. Over a third of elderly patients have been found to intentionally alter their medication regimens. This has both positive and negative consequences.

Efforts to increase compliance among some patient groups may not beneficial in terms of either health gain or NHS cost saving. There is evidence that over half of women prescribed HRT take less than 75% after 1 year. Concerns about adverse effects and risks to health have been shown to be a prime concern amongst these women. A recent review concluded that HRT increases risks of breast cancer, stroke and pulmonary embolism.