To evaluate the stability of essential drugs stored in realistic tropical conditions, we have carried out a two-year prospective study in western Burkina Faso. Twenty-seven essential drugs were stored in a rural site and a urban one where temperature and hygrometry were recorded daily. Samples of each drug were taken for further analysis to the World Health Organization Collaborative Center for the Study of Stability of Drugs in Nantes, France every three months. Quantitative analysis showed that the majority of samples suffered no significant loss of their active ingredient. In contrast, ampicillin, erythromycin, sulfaguanidine, injectable furosemide, penicillin G, trimethoprim, and chloroquine showed more than a 10% quantitative loss of their active ingredient. Thus, it is not recommended that these essential drugs be stored for more than one year in a tropical climate.