Background: Adverse effects of medications are an important cause of morbidity and hospital admissions. Errors in prescription or preparation of medications by pharmacy personnel are a factor that may influence these occurrence of the adverse effects Aim: To assess the frequency and type of errors in prescriptions and in their preparation at the pharmacy unit of a regional public hospital. Material and methods: Prescriptions received by ambulatory patients and those being discharged for the hospital, were reviewed using a 12 item checklist. The preparation of such prescriptions at the pharmacy unit was also reviewed using a seven item checklist. Results: Seventy two percent of prescriptions had at least one error. The most common mistake was the impossibility of determining the concentration of the prescribed drug. Prescriptions for patients being discharged from the hospital had the higher number of errors. When a prescription had more than two drugs, the risk of error increased 2.4 times. Twenty four percent of prescription preparations had at least one error. The most common mistake was the labeling of drugs with incomplete medical indications. When a preparation included more than three drugs, the risk of preparation error increased 1.8 times. Conclusions: Prescription and preparation of medication delivered to patients had frequent errors. The most important risk factor for errors was the number of drugs prescribed.Background: Adverse effects of medications are an important cause of morbidity and hospital admissions. Errors in prescription or preparation of medications by pharmacy personnel are a factor that may influence these occurrence of the adverse effects Aim: To assess the frequency and type of errors in prescriptions and in their preparation at the pharmacy unit of a regional public hospital. Material and methods: Prescriptions received by ambulatory patients and those being discharged for the hospital, were reviewed using a 12 item checklist. The preparation of such prescriptions at the pharmacy unit was also reviewed using a seven item checklist. Results: Seventy two percent of prescriptions had at least one error. The most common mistake was the impossibility of determining the concentration of the prescribed drug. Prescriptions for patients being discharged from the hospital had the higher number of errors. When a prescription had more than two drugs, the risk of error increased 2.4 times. Twenty four percent of prescription preparations had at least one error. The most common mistake was the labeling of drugs with incomplete medical indications. When a preparation included more than three drugs, the risk of preparation error increased 1.8 times. Conclusions: Prescription and preparation of medication delivered to patients had frequent errors. The most important risk factor for errors was the number of drugs prescribed.