Objective: To analyze the excipients and impurities contained in the various docetaxel products available on the market and find out whether they may be responsible for any of the different adverse events associated with the use of docetaxel in patients with breast cancer receiving adjuvant or neoadjuvant treatment. Method: This is a prospective, multicenter, longitudinal observational, study carried in 26 hospitals in Madrid, Catalonia, Andalusia, and the Valencia Region. The different docetaxel formulations were characterized in terms of their pH, amount of the active ingredient and impurities. The cumulative incidence of adverse events of any grade was evaluated. Adverse events were stratified by drug type and differences were analyzed by means of a chi-square test. Results: Statistically significant differences were found between the different docetaxel formulations in the cumulative per-cycle incidence of: dosage change, anemia, hypersensitivity reactions and anaphylaxis, neuropathy, palmoplantar and dermal toxicity, ungual toxicity and facial edema. The formulation with the lowest content of impurities showed better results in terms of change of dosage, visits to the emergency room and incidence of anemia and facial edema. However, it was associated with poorer results regarding hospitalization, febrile neutropenia, motor neuropathy and palmoplantar toxicity. Conclusions: The results of the study showed differences in the incidence of adverse events of the different docetaxel products available in Spain. Such differences were statistically significant for some of the variables analyzed. The study was not able to determine which of the products offered the best toxicity profile. Nor was it possible to establish a correlation with respect to the composition of excipients or the content of impurities.