The aim of the study was to check the association of the Morisky Medication Adherence Scale with the prescription complexity and with the patient’s level of knowledge about the prescription. A cross-sectional study was conducted by means of an interview and analysis of the prescription received by patients selected at Family Health Strategy Units. A Poisson regression model with robust variance was used to describe the association between the variables. Low adherence was significantly associated with highly complex prescriptions: Prevalence Ratio (PR) = 1.53; 95 % Confidence Interval (CI) = 1.28-1.82, in comparison with low and moderate complexity. Low adherence was also significantly associated with very low levels of knowledge: regarding the highest level (third tertile), the first and second tertiles had PR = 4.44; 95 % CI = 3.26-6.06 and PR = 3.22; 95 % CI = 2.33-4.47, respectively. The Morisky scale is useful to measure low adherence, with the advantage of being an easily understood instrument, which can be quickly administered, presenting simple interpretation and application