OBJECTIVES: To determine whether the formation of Drugs and Therapeutics Committees (DTC’s) to implement Rational Drug Use Policies has impacted positively on prescribing patterns at Ussher Polyclinic. METHODS: The double differencing method was employed in this study. Data for the period 1990 – 2009 were collected from prescription sheet of patients attending to the outpatient department of Ussher polyclinic and a private clinic. Data was analyzed using Microsoft Office Excel 2007. Fourteen thousand four hundred prescriptions sheets were analyzed from both clinics using systematic random sampling. Variations of rational prescribing indicators from their optimal values (as determined by WHO standards) were determined and the mean values used in the determination of the difference- in -difference estimate. Subtracting the change in prescribing indicator of the private clinic without a DTC over the two time periods( 1990 – 2000 and 2000 – 2009) from the change observed at Ussher polyclinic with a functioning DTC  from the year 2000, over the same time periods(1990-2000 and 2000-2009) gives the Difference-in-Difference estimate. RESULTS: The Difference-in-Difference (DiD) estimate showed that reduction in average number of drugs prescribed was 0.63 units lower at Ussher polyclinic compared to the private clinic after the post-intervention period. The use of antibiotics was 8.72% lower at Ussher Polyclinic as compared to the private hospital. Reduction in injection use was 25.76% lower at Ussher Polyclinic compared to the private clinic. Improvement in generic prescribing was 44.62% higher at Ussher Polyclinic compared to the private clinic. Increased adherence to prescribing according to Essential Drug List was 43.74% higher at Ussher Polyclinic compared to the private clinic. CONCLUSIONS: The overall picture suggests that implementation of rational drug use policies have impacted positively on prescribing behavior at Ussher Polyclinic with significant changes between the pre-intervention and post-intervention periods.