OBJECTIVES To identify and describe misunderstandings between patients and doctors associated with prescribing decisions in general practice.   
 DESIGN Qualitative study.   
 SETTING 20 general practices in the West Midlands and south east England.   
 PARTICIPANTS 20 general practitioners and 35 consulting patients.   
 MAIN OUTCOME MEASURES Misunderstandings between patients and doctors that have potential or actual adverse consequences for taking medicine.   
 RESULTS 14 categories of misunderstanding were identified relating to patient information unknown to the doctor, doctor information unknown to the patient, conflicting information, disagreement about attribution of side effects, failure of communication about doctor's decision, and relationship factors. All the misunderstandings were associated with lack of patients' participation in the consultation in terms of the voicing of expectations and preferences or the voicing of responses to doctors' decisions and actions. They were all associated with potential or actual adverse outcomes such as non-adherence to treatment. Many were based on inaccurate guesses and assumptions. In particular doctors seemed unaware of the relevance of patients' ideas about medicines for successful prescribing.   
 CONCLUSIONS Patients' participation in the consultation and the adverse consequences of lack of participation are important. The authors are developing an educational intervention that builds on these findings.
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