Background: Medication errors are important in chemotherapy centers, which can cause excessive morbidity and extra cost for patients because of the high toxicity and low therapeutic index of antineoplastic agents. Using standardized forms for prescription and administration of medications is one of the ways of reducing medication errors in the chemotherapy process. Objectives: In the current study, the authors aimed to evaluate the effects of the standard forms implementation and detect medication error and adverse drug event (ADE) rates involving in chemotherapy regimens used in breast cancer in the public/private outpatient chemotherapy settings. Methods: A cross-sectional interventional study was performed prospectively at two adult outpatient cancer centers, Mashhad, Iran. To avoid errors, a standardized order sheet was established to document information regarding breast cancer chemotherapy. The effect of the standard sheet on decreasing errors in ordering and administering was evaluated. The epidemiology of the errors and adverse drug events was reported. Results: Of the 217 visits (164 at a public hospital and 70 at a private clinic) of 84 adult patients (64 at the hospital and 20 at the clinic) involving 385 medications, 41% were associated with a medication error. Of these errors, 5% occurred in the private clinic compared to 95% of the errors occurring in the public hospital. A standardized approach helped to reduce errors in the selection of the regimen type. However, physicians did not calculate doses based on the standard sheets so the most common error type was improper dose prescription (38.2% of the 89 cited error types). The effect of standard sheets in the administration phase could not be assessed due to the incomplete data presented by nurses. 62% of the errors originated in the prescription phase of medication and 33% originated in the administration phase. The ADE rate was 9.6% but no life-threatening adverse drug event was recorded. Conclusions: Based on the current study, the medication errors occurred more commonly in the public setting and the prescribing errors were the most common ones. Standardized order sheets would be very beneficial in minimizing the medication errors if are used accurately.