RATIONALE, AIMS, AND OBJECTIVES: Misunderstanding medication dosage regimen instructions can lead to unintentional misuse of a prescribed medicine, non-adherence to providers' instructions, and other treatment-related issues. We aimed to evaluate the frequency of and factors associated with older patients' misunderstanding of medication dosage regimen instructions after consultation with a general practitioner. METHOD: This cross-sectional study was conducted in 22 primary-care facilities in Brazil. Data were collected from September 2016 to December 2017 using a multidimensional questionnaire. Patients who were 60 years old or older who visited primary care units were included in the study (n = 416). RESULTS: Of the older patients interviewed, 38.2% had a misunderstanding of medication dosage regimen instructions; being female was a protective factor against the misunderstanding of medication dosage regimen instructions (prevalence ratio [PR] = 0.63; 95% confidence interval [CI] = 0.45-0.89). In relation to other factors with an important association, misunderstanding medication dosage regimen instructions was 71% higher among illiterate participants (PR = 1.71; 95% CI = 1.25-2.35), 39% higher among people who considered their memory to be poor (PR = 1.39; 95% CI = 1.01-1.91), 49% higher in those who did not have a job at the time of the interview (PR = 1.49; 95% CI = 1.01-2.19), and 50% higher in patients who had been prescribed five or more medications (PR = 1.50; 95% CI = 1.02-2.20). CONCLUSIONS: The results showed that older people's misunderstandings of medication dosage regimen instructions after consultation with a general practitioner was greater than expected due to a range of factors, especially polypharmacy, poor literacy, poor memory, and having a job at the time of the interview. Health services and professionals should implement strategies to increase the quality of the guidance given to elderly individuals and to ensure their adherence to the regimen instructions of their medications.