1. Patients and their carers will usually be the first to notice any observable problems resulting from medication errors. They will probably be unable to distinguish between medication errors, adverse drug reactions, or 'side effects'. 2. Little is known about how patients understand drug related problems or how they make attributions of adverse effects. Some research suggests that patients' cognitive models of adverse drug reactions bear a close relationship to models of illness perception. 3. Attributions of adverse drug reactions are related to people's previous experiences and to their level of education. The evidence suggests that on the whole patients' reports of adverse drug reactions are accurate. However, patients do not report all the problems they perceive and are more likely to report those that they do perceive as severe. Patients may not report problems attributed to their medications if they are fearful of doctors' reactions. Doctors may respond inappropriately to patients' concerns, for example by ignoring them. Some authors have proposed the use of a symptom checklist to elicit patients' reports of suspected adverse drug reactions. 4. Many patients want information about adverse drug effects, and the challenge for the professional is to judge how much information to provide and the best way of doing so. Professionals' inappropriate emphasis on adherence may be dangerous when a medication error has occurred. 5. Recent NICE guidelines recommend that professionals should ask patients if they have any concerns about their medicines, and this approach is likely to yield information conducive to the identification of medication errors.