Introduction: Pharmacotherapy is the main therapeutic resource for the management of diseases. However, the number of drugs prescribed, dose frequency, and mode of administration can make the treatment more complex and influence treatment outcomes. The aim of this study was to measure the complexity of prescribed medication regimens in primary health care (PHC) services in Ribeirão Preto, Brazil.Methods: This cross-sectional study included 1,009 participants: 889 from primary health units and 120 from family health units in Ribeirão Preto, Brazil. Treatment complexity was assessed using the Medication Regimen Complexity Index (MRCI).Results: MRCI mean scores were 12.5 points (SD = 9.3) and dose frequency was the major contributor to increase the score. The complexity of pharmacotherapy showed a significant correlation with the number of prescribed medications (r = 0.93, p < 0.01), but not with patients' age (r = 0.28, p < 0.01). There is also no difference in complexity between the sexes (p = 0.83) and the types of primary health care service (p = 0.31). An analysis of variance revealed that patients with lower levels of education receive more complex prescriptions (p < 0.01).Conclusions: The pharmacotherapy prescribed in PHC services from Ribeirão Preto, Brazil is complex, and there is a need to concentrate efforts and adopt strategies to simplify drug prescription without compromising patient's clinical status.Keywords: Primary health care; drug prescriptions; drug therapy; medication regimen complexityIntroduction: Pharmacotherapy is the main therapeutic resource for the management of diseases. However, the number of drugs prescribed, dose frequency, and mode of administration can make the treatment more complex and influence treatment outcomes. The aim of this study was to measure the complexity of prescribed medication regimens in primary health care (PHC) services in Ribeirão Preto, Brazil.Methods: This cross-sectional study included 1,009 participants: 889 from primary health units and 120 from family health units in Ribeirão Preto, Brazil. Treatment complexity was assessed using the Medication Regimen Complexity Index (MRCI).Results: MRCI mean scores were 12.5 points (SD = 9.3) and dose frequency was the major contributor to increase the score. The complexity of pharmacotherapy showed a significant correlation with the number of prescribed medications (r = 0.93, p < 0.01), but not with patients' age (r = 0.28, p < 0.01). There is also no difference in complexity between the sexes (p = 0.83) and the types of primary health care service (p = 0.31). An analysis of variance revealed that patients with lower levels of education receive more complex prescriptions (p < 0.01).Conclusions: The pharmacotherapy prescribed in PHC services from Ribeirão Preto, Brazil is complex, and there is a need to concentrate efforts and adopt strategies to simplify drug prescription without compromising patient's clinical status.Keywords: Primary health care; drug prescriptions; drug therapy; medication regimen complexity