OBJECTIVES: To discuss the importance of published evidence to modify prescribing behaviours and to describe the case of metronidazol in suspected intestinal amoebiasis. METHODS: In Mexico and developing countries intestinal, amoebiasis is over diagnosed in acute diarrhoea (Entamoeba histolytica found only in 2%) with the consequent over use of metronidazol (40-60%). We reviewed 90,700 prescriptions during August 1999 in three primary care facilities in Mexico City. A total of 1790 prescriptions of oral metronidazol were found and 300 were randomly chosen to review the medical charts (diagnosis and dosage characteristics). Micro-costing was also performed using the prices of the institution. RESULTS: Metronidazol was prescribed when intestinal amoebiasis was suspected in 746 cases (83%). The prescriptions did not follow the established criteria (WHO and other authors) in 96% (n = 724). Metronidazol was prescribed when there was diarrhoea with mucus and blood in 57% and in unspecified data of "colitis" in 39%. Only in 8 cases asymptomatic amoebiasis was diagnosed. Mean monthly cost per medical office of unjustified metronidazol prescription was $43.5 Mexican pesos (4 USD). The national annual projection of that institution was $3,495,887 (322,350 USD). Sensitivity analysis for the worst scenario (3% of metronidazol prescriptions, 96% unjustified prescriptions and a higher unit price) would lead to an annual cost of $5,193,064 (478,844 USD). CONCLUSIONS: Inadequate prescribing criteria persists for metronidazol despite of clinical criteria and published evidences. Our study showed that there is a gap between medical practice and clinical and health economics research. There is also an opportunity to update clinicians' knowledge using educational strategies. Another challenge is to educate patients in the sense that drugs prescription is not always the best option but hygiene, good sanitary conditions and nutrition, and prevention. Finally the opportunity cost of unjustified prescriptions should not be underestimated but more often evaluated and taken into account for policies.