OBJECTIVES: To calculate the cost savings if the cheapest trade marks were prescribed. Competence among drug companies has contributed to a rising potential of saving, so making possible a more efficient use of drugs. METHODS: We take a database with every prescription made in 1998 by a staff of 239 health-system physicians in Cádiz-Bahía Primary Care Area. This Area has 468,908 inhabitants. The study was done by taking a sample of those drugs with more potential of cost saving: 6 antiulcer drugs, 10 antibiotics, 8 antihypertensive and 9 NSAIDs. RESULTS: The real cost of prescription of these drugs in 1998 was 3.583.500E. 790.841E (28,8%) could be saved if cheaper trade marks were prescribed. That proportion was similar among anti-hypertensives (30,5%) NSAIDs (29,5%), antiulcer drugs (29,4%) and antibiotics (24,5%), but potential savings were greater for antihypertensives (414.762E) than for antiulcer drugs (232.573E), antibiotics (196.297E) or NSAIDs (187.265E) . Related to specific drugs, 10 drugs get more than 80% of potential savings, and only 4 get more than 50%. They are enalapril (23,5%, 268919E), omeprazol (12%,138.000E), captopril (10,2%, 116.724 E), ibuprofen (8,2%, 93.459E). Among antibiotics, we could reach 79% of potential savings with ciprofloxacin and amoxicillin/clavulanate. 93% of potential cost savings in antihypertensives could be reached with enalapril and captopril. With omeprazole only we could reach 59% of savings in antiulcer drugs. There are not such differences among NSAIDs. CONCLUSIONS: A great proportion (25-30%) of costs could be saved if the physician knows and prescribes the cheapest alternatives among those with equal content. Special attention is needed about potential cost savings of few drugs.