OBJECTIVES: Alginates are often used first line for gastroesophageal reflux disease (GORD).  In the UK many primary care trusts (PCTs) believe alginates to be interchangeable and discriminate between products solely on cost.  Consequently several PCTs have implemented alginate substitution initiatives at the level of the prescriber in an attempt to save costs. We sought to evaluate whether PCT guidance suggesting alginate interchangeability was reflected in a large longitudinal representative UK database by comparing alginate switching patterns. METHODS: IMS Disease Analyzer data was retrieved in which patients commencing alginate monotherapy were followed for one year from treatment initiation. From the dataset a cohort of 3367 patients commencing therapy between June 1, 2005 and May 31, 2006 were tracked following their first alginate prescription.  Switching patterns were analysed using chi-square and proportions tests for the leading alginate brands: Gaviscon Advance (GA) and Peptac.   RESULTS: During the observation period 2238 and 1129 subjects commenced GA and Peptac, respectively. After one year 87% of people on GA compared with 83% on Peptac were maintained on their initial alginate (p= 0.013). Similar switching rates were found when excessive switchers, defined as more than 5 different interventions in one year, were removed from the dataset (p=0.04). Significantly higher proportions of subjects on Peptac (4%) switched to an alternative alginate compared with those on GA (2%) (p<0.005). The frequencies of observed switching at the end of 12 months was different for those started on GA and Peptac in the Pearson’s test of independence (p=0.039). Furthermore, subjects started on combination GA + proton pump inhibitor (PPI) were more likely to discontinue PPI at 12 months compared with those started on Peptac + PPI at 17% and 8%, respectively (p=0.016). CONCLUSIONS:  Significant differences in treatment failure were observed for the leading UK alginates products.  Additionally, patients initiated on GA + PPI were more likely to discontinue their PPI compared with persons treated with PPI + Peptac. These results highlight the importance of considering a broader range of costs and treatment factors impacting patients when introducing product substitution policies.