Nonadherence to prescribed pain medication is common in chronic noncancer pain patients. Beliefs about pain medication have been reported to be associated with nonadherence behavior in cross-sectional studies. The aim of this study was to prospectively investigate the relationship between patients' beliefs about pain medication and their medication adherence and treatment outcome. Chronic noncancer pain patients completed a baseline questionnaire including the 47-item Pain Medication Attitudes Questionnaire, consisting of 7 subscales with regards to beliefs on prescribed medication. After 11 weeks, medication underuse and overuse were assessed by self-report. In addition, patient satisfaction with regards to the effect of prescribed pain medication and satisfaction as regards prescribed medication and care, and the burden of side effects were assessed. A total of 133 participants completed the baseline questionnaire, and 99 patients completed the follow-up questionnaire. Concerns over side effects at baseline were positively associated with underuse and the presence of side effects after 11 weeks. The perceived need was positively associated with overuse, whereas concerns over addiction were negatively associated with overuse. Concerns over tolerance were negatively associated with patient satisfaction with medication effects after 11 weeks. Concerns over tolerance and mistrust in the prescribing doctor were negatively associated with satisfaction about medication and care. Attitudes and concerns toward pain medication are associated with adherence patterns and outcome parameters. To improve medication adherence and therapy outcome, patient beliefs about pain medication should be taken into account by providing tailored education, adequate follow-up, or alternate therapy.