OBJECTIVES: Medication adherence has been shown to be an important factor in preventing schizophrenia-related relapses, and long-acting injectable risperidone has been shown to be effective in improving medication adherence.  The objective of this study was to determine the medication adherence rates of schizophrenic patients treated with either oral risperidone only (oral cohort) or long-acting injectable risperidone +/- oral risperidone (injectable cohort).  METHODS: Texas Medicaid prescription claims data (January 2006 to December 2008) were analyzed for patients with ICD-9CM codes for schizophrenia (295.xx).  Records were assessed for a period of 18 months (6-month pre-index period with no antipsychotics plus 12-month post-index period).  A medication possession ratio (MPR) of ≥ 80% or a proportion of days covered (PDC) of ≥ 80% was considered adherent.  RESULTS: A total of 1,470 patients met inclusion criteria; 31% (n=462) had an MPR ≥ 80%.  Based on MPR, 30% (405/1338) in the oral cohort and 43% (57/132) in the injectable cohort were adherent.  Logistic regression analyses adjusting for demographic covariates (age, gender, race) showed that patients in the injectable cohort were about 77% more likely to be adherent based on MPR (OR=1.77, 95% CI=1.22-2.56, p<0.05) compared to the oral cohort.  Of the demographic covariates, race (Blacks = less adherent; p<0.001) was the only significant covariate.  Similarly, increased adherence in the injectable cohort (OR=1.81, 95% CI=1.24-2.63, p<0.05) was also found using PDC, where race (Blacks = less adherent; p<0.001) and gender (women = less adherent; p<0.05) were significant.  CONCLUSIONS: Overall, less than one-third of all patients were adherent (MPR ≥ 80%) one year after their index date.  Patients taking long-acting injectable risperidone had higher rates of adherence compared to patients taking oral risperidone alone.  In addition, this finding was confirmed by PDC, which has been shown to be a more conservative calculation of adherence in patients with schizophrenia.