Increasing concerns regarding access to and appropriateness of medicinal drug use have led many governments in developing countries to develop national policies and regulations intended to increase the affordability, supply, safety, and rational use of pharmaceuticals. However, little is known about the intended and unintended impacts of these social experiments on actual drug use. We conducted a critical review and synthesis of the international literature in an attempt to define the current state of knowledge regarding drug policy effects on drug use, and to extract from the evidence important lessons for future policy and research. Literature sources included the archives and computerized databases, articles published in medical and pharmacy journals, as well as published annotated bibliographies. The evaluated interventions included three broad categories: (1) multi-component national drug policies including essential drug programs; (2) drug supply and cost-sharing programs; and (3) regulatory measures. Most of these studies utilized weak research designs that evaluated programs solely on the basis of post-intervention measures. Only two studies measured pre-policy utilization, but did not include a control group. Thus, none of the results are conclusive, and the findings represent, at best, hypotheses for more rigorous studies of policy impacts. Some suggestive findings include an association between increases in the supply of essential drugs (combined with training) and more appropriate use of medications in primary care settings. In addition, preliminary data suggest some unintended effects of de-registration of drugs or upward reclassification of specific medicines. Similarly, loosening restrictions have sometimes been accompanied by increased dispensing of specific drugs by unqualified personnel. The available studies focused only on a few categories of national and regulatory policies. Because of poor study design, the results do not provide valid data to determine whether national drug policies improve drug use. Moreover, no studies have evaluated the effects of major and recent changes, such as increased use of product patents, national pharmaceutical insurance policies, and increased privatization of pharmaceutical products and services. Future studies need to explore the consequences of these emerging developments on drug access and use. Despite the difficulties inherent in evaluation of national policies, stronger research designs can and should be carried out. Interrupted time-series analysis and other more rigorous designs should become standard designs for policy evaluation in the same way that standard treatment guidelines are intended to guide medical practice.