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Background: The lack of adherence to drug therapy is a major problem; it can contribute to significant deterioration of disease and increased health-care costs. Improving medication adherence is a big challenge; there is no simple solution to the problem. It is thus essential to improve our knowledge of non-adherence (NA) and its causes.

Aims: The aims of the thesis were to study the influence of gender and psychological distress on self-reported, intentional and unintentional non-adherent behaviour, and to investigate the reasons for NA.

Methods: A population-based study that included a postal questionnaire was carried out in a cross-section of the general Swedish population (n=7,985, aged 18-84 years). The response rate was 61.1% (n=4,875) and current prescription drug use was reported by 2,802 participants. The questionnaire covered use of prescription drugs, NA to the drug regimens, reasons for NA, economic status, attitudes to drugs, and the presence of somatic or mental problems, and also included the Hospital Anxiety and Depression Scale questionnaire.

Results: The results showed differences in various self-reported non-adherent behaviour patterns and reasons for NA between the genders. In most cases, these remained after controlling for confounders such as socioeconomic factors and attitudes to drugs that are known to differ between women and men. Associations were also found between symptoms of anxiety and/or depression and the presence of intentional or unintentional non-adherent behaviour (with a stronger average association for intentional NA), and between anxiety/depression and some of the reasons given for NA, e.g. adverse drug reactions (ADRs).

Conclusions: Although it was not possible to confirm causal relationships, this thesis emphasises the effects of gender and psychological distress on NA. In summary, both gender and anxiety and/or depression influenced non-adherent behaviour and the reasons given for NA. For instance, ADRs seemed to influence the decision not to take the drug as prescribed, especially among women and participants under psychological distress. It is suggested that a deep understanding of the causes of NA and of the impact of gender and psychological distress on the outcomes would help those aiming to improve adherence to prescribed medication.

Background Drug use has increased over recent decades, and is especially great among women and among people with mental health problems. To take advantage of the full potential of drugs and to avoid drug-related problems, drug prescription needs to be correct and the drugs need to be taken according to the prescribed regimens. Research on drug utilization is thus important to the public health.

Aim To study the influence of gender and depression on drug utilization, prescription of drugs and self-reported use of drugs, i.e. adherence.

Methods The thesis included two population-based questionnaires and data from the Swedish Prescribed Drug Register (SPDR) covering Swedish citizens 18-84 years. The questionnaire in Study I and II included items on prescription drug use and adherence to treatment regimens; Study II also included the Hospital Anxiety and Depression Scale (HADS) for self-estimation of anxiety/depression. The questionnaire in Study III included the HADS and data from the SPDR on prescribed antidepressants. Study IV included data from the SPDR on all types of prescribed drugs.

Results Men and women differed in non-adherent behaviours and reasons for non-adherence, for example, men were more likely to report forgetting to take the drug, while women were more likely to report adverse drug reactions (ADRs) as a reason for non-adherence. Further, both anxiety and depression were associated with non-adherence and with ADRs as a reason for non-adherence. In addition, men reported depression to a greater extent than women did but used antidepressants to a lesser extent, while women used antidepressants without reporting depression more often than men did, which may be a sign of under-treatment among men and over-treatment among women. Moreover, the associations between antidepressants and other types of drugs differed by gender; they were often specific, or stronger, in women than in men, which may be a sign of a gender difference in comorbidity between depression and other conditions.

Conclusions Although the cross-sectional study design prevented confirmation of causality, the thesis found that gender and depression influence both prescription of drugs and adherence, and are thus important to pay attention to in clinical practice as well as research.