Despite the widespread prescription of medicines, patients' knowledge about the drugs they take is limited. Many people feel that not enough is explained by doctors and pharmacists, but even when information is given verbally it is often forgotten or misunderstood. One solution might be to provide information leaflets with prescribed medicines. Although patients would welcome additional written information, well designed patient-oriented leaflets have not been available in the UK until recently. Moreover, there have been few systematic studies which have examined the effects of leaflets in this country. This thesis describes the development of Prescription Information Leaflets (PILs) and an evaluation of their effects in general practice. A two-sided 'details plus summary' PIL format was developed, which conferred advantages in terms of improved knowledge and satisfaction over a one-sided 'details only' version. A scries of generic PILs were produced for six commonly prescribed groups of medicines: non-stcroidal anti-inflammatory drugs (NSAIDs), penicillins, /8-adrenoceptor antagonists, inhaled bronchodilators, diuretics and bcnzodiazepincs. The leaflets were constructed according to guidelines for the design of technical information and contained information on how to take the medicine, its common side-effects and advice about safe storage and disposal. The PILs were evaluated in six small Hampshire towns. Leaflets were given to 671 patients in four of the towns by either a pharmacist or a general practitioner. A further 547 patients in the two control towns received no leaflets. Those who received leaflets knew more about their medicines, particularly the side-effects. However, this knowledge was not associated with spurious side-effects, except in the case of benzodiazepines. Although some were worried at the prospect of side-effects, almost all patients thought that these leaflets were a good idea. An important finding was that patients who received leaflets were significantly more satisfied with the information they received about their medicine. In addition, initial improvements in knowledge and satisfaction were apparent 12 months after the original prescription. It is concluded that the benefits of PILs outweigh the disadvantages and justify the use of leaflets on a routine basis. However, since general practitioners and pharmacists had difficulty remembering to issue the leaflets, it is suggested that they should be incorporated in original packs by the pharmaceutical manufacturer.