Low adherence of patients to prescribed, self-administered medical interventions is ubiquitous. Low adherence limits the benefits of current medical care. Efforts to assist patients to follow treatments might improve the efficiency of care and substantially enhance benefits. Our objective was to summarise the results of randomised controlled trials (RCTs) of interventions to help patients follow prescriptions for medications. A previous systematic review was updated through computerised searches in Medline, International Pharmaceutical Abstracts, Psychinfo, and HSTAR online databases; bibliographies in articles on patient adherence; articles in the reviewers' personal collections; and contact with authors. Articles were judged of interest if they reported original data concerning an unconfounded RCT of an intervention to improve adherence with prescribed medications, with one or more measure of medication adherence, one or more measure of treatment outcome, at least 80% follow-up of each group studied, and, for long-term treatments, at least 6 months of follow-up for studies with positive initial findings. Information on study design features, interventions and controls, and findings were extracted by one reviewer (RK) and checked by the other two reviewers. 1,553 relevant citations and abstracts were screened, 252 full text articles were reviewed in detail, and 13 RCTs met all criteria. The studies were too disparate in clinical problems, adherence interventions, measures and reporting of adherence, and the clinical outcomes studied to warrant meta-analysis. Seven of 15 interventions were associated with improvements in adherence and six interventions led to improvements in treatment outcomes. For short-term treatments, one study showed an effect on adherence and outcome of counselling and written information. The interventions that were effective for long-term care were complex, including various combinations of more convenient care, information, counselling, reminders, self-monitoring, reinforcement, family therapy, and other forms of additional supervision or attention. Even the most effective interventions did not lead to substantial improvements in adherence. Although adherence and treatment outcomes can be improved by certain-usually complex-interventions, full benefits of medications cannot be realised at currently achievable levels of adherence. It is time that additional efforts be directed towards developing and testing innovative approaches to assist patients to follow treatment prescriptions.