OBJECTIVES:  Examine how changes in the medication possession ratio (MPR) affect patient outcomes among multiple sclerosis (MS) patients treated with glatiramer acetate (GA; Copaxone®). METHODS: Data were obtained from i3 InVision Data Mart for january 1, 2006 – March 31, 2010.  Patients were included if they were diagnosed with MS, initiated therapy with GA, and had continuous insurance coverage from 6 months prior through 24 months post initial use of GA (N=839).  Multivariate regressions which controlled for patient characteristics examined the association between achievement of alternative MPR goals and patient relapses and charges.    Logistic regressions were used to examine the relapses, while generalized linear models were used to examine charges.  RESULTS:   Patients who achieved an MPR of at least 0.7 had significantly lower odds of relapse, with achievement of a threshold of 0.7, 0.8 or 0.9, respectively, associated with an odds ratio of relapse of 0.545 (95% CI 0.351 – 0.824), 0.530 (95% CI 0.371 – 0.870), and 0.421 (95% CI 0.260 – 0.679).  Larger reductions in total direct medical charges, excluding drugs, were seen with higher MPR thresholds,  For example, achievement of an MPR threshold of at least 0.5 was associated with $1524 lower total charges (P=0.0034), while a MPR threshold of 0.90 was associated with $1825 lower charges (P=0.0005).  Examining MS-related total charges, exclusive of drugs, revealed that a MPR threshold of 0.90 was associated with $986 lower total MS-related charges (P=0.0498).  Results also found an association between patient adherence to GA and statistically significant reduction in inpatient, ER, outpatient, MS-related inpatient and MS-related outpatient charges.  CONCLUSIONS:  As adherence improves the odds of relapse decreases and cost-savings, exclusive of drugs, generally increase.  Results suggest that, despite the higher costs associated with increased usage of GA, patient outcomes are improved and there are cost-offsets associated with “compliant” use of the medication.