BACKGROUND: Limited information exists in regard to drug omissions and unjustified medications in the hospital discharge summary (DS). OBJECTIVE: To evaluate the incidence and types of drug omissions and unjustified medications in the DS, and to assess their potential impact on patient health. METHODS: A prospective observational review of the DSs of all patients discharged from our Internal Medicine Department over a 3-month period. Data assessment was made by internists using a structured form. RESULTS: Of the 577 evaluated DSs, 66% contained at least one inconsistency accounting for a total of 1012 irregularities. There were 393 drug omissions affecting 251 patients, 32% of which were potentially harmful. Seventeen per cent of all medications (619/3691) were unjustified, affecting 318 patients. The unjustified medication was potentially harmful in 16% of cases, occurred significantly more frequent in women than in men (61% vs 50%; p = 0.008) and increased linearly with the number of drugs prescribed (p<0.001). Drug omission had a twofold higher potential to cause harm than unjustified medication. CONCLUSIONS: Drug omissions and unjustified medications are frequent, and systemic changes are required to substantially reduce these inconsistencies.