AIMS: To evaluate oxidative stress parameters in patients with type 2 diabetes mellitus treated with metformin, relating these values to its side effects, plasma levels, glycemic control, diabetic complications, lipid profile, and the influence of pharmacotherapeutic follow-up. METHODS: Patients with type 2 diabetes mellitus, on metformin and in pharmacotherapeutic follow-up for four months, were evaluated. The pharmacotherapeutic follow-up consisted in providing information and answering patients' questions about medication and disease. In addition, administration times, dosages, and presence or absence of side effects related to the use of metformin were verified. Glycemic and lipid profile, oxidative stress (superoxide dismutase and malondialdehyde) and plasma metformin were evaluated. Pearson's correlation and Spearman's correlation were performed to evaluate the relationship between the variables at the beginning of the study. The independent t-test and Mann-Whitney U test were used to assess the difference between the groups with and without diabetic complications. The range of values between the beginning and end of the study was evaluated using Student's t-test or Wilcoxon U test. The significance level was set at 5%. RESULTS: The initial sample consisted of 49 patients aged 59±9 years with a body mass index of 29.8±5.1 kg/m 2 , who have had diabetes for a median time of 36 months (interquartile range of 1-240) and have been on metformin for a median time of 36 months (interquartile range of 1-180). Twenty-five patients left the study between the second and fourth meetings. Malondialdehyde levels differed between before and after pharmacotherapeutic follow-up, being positively correlated with blood glucose, glycohemoglobin, and triglyceride level, and negatively correlated with metformin and superoxide dismutase. Blood glucose, glycohemoglobin, and malondialdehyde levels increased, whereas metformin levels decreased in the group with diabetic complications, and there was a correlation between malondialdehyde and the number of diabetic complications per patient. CONCLUSIONS: In this sample of patients with type 2 diabetes mellitus treated with metformin, oxidative stress was more pronounced in those with poor glycemic control and diabetic complications.KEY WORDS: diabetes mellitus; oxidative stress; superoxide dismutase; malondialdehyde; metformin; diabetic complications. RESUMOOBJETIVOS: Avaliar parâmetros de estresse oxidativo em pacientes com diabetes mellitus tipo 2 em uso de metformina, relacionando estes valores a seus efeitos adversos, níveis plasmáticos, controle glicêmico, complicações diabéticas, perfil lipídico, e a influência do acompanhamento farmacoterapêutico. MÉTODOS: Foram avaliados pacientes com diabetes mellitus tipo 2, em uso de metformina, em acompanhamento farmacoterapêutico por quatro meses. O acompanhamento farmacoterapêutico consistiu na prestação de informações e no esclarecimento de dúvidas dos pacientes sobre a medicação e a doença. Além disto, foram verificados os horá...