Background In Belgium, a co-insurance system is applied in which patients pay a portion of the cost for medicines, called co-payment. Co-payment is intended to make pharmaceutical consumers more responsible, increase solidarity, and avoid or reduce moral hazards. Objective Our objective was to study the possible influence of co-payment on sales volume and generic market share in two commonly used medicine groups: cholesterol-lowering medication [statins (HMG-CoA reductase inhibitors) and fibrates] and acid-blocking agents (proton pump inhibitors and histamine H2 receptor antagonists). Methods and Data The data were extracted from the Pharmanet database, which covers pharmaceutical consumption in all Belgian ambulatory pharmacies. First, the proportion of sales volume and costs of generic products were modelled over time for the two medicine groups. Second, we investigated the relation between co-payment and contribution by the national insurance agency using change-point linear mixed models. Results The change-point analysis suggested several influential events. First, the generic market share in total sales volume was negatively influenced by the abolishment of the distinction in the maximum co-payment level for name brands and generics in 2001. Second, relaxation of the reimbursement conditions for generic omeprazole stimulated generic sales volume in 2004. Finally, an increase in co-payment for generic omeprazole was associated with a significant decrease in omeprazole sales volume in 2005. The observational analysis demonstrated several changes over time. First, the co-payment amounts for name-brand and generic drugs converged in the observed time period for both medicine groups under study. Second, the proportion of co-payment for the total cost of simvastatin and omeprazole increased over time for small packages, and more so for generic than for name-brand products. For omeprazole, both the proportion and the amount of co-payment increased over time. Third, over time the prescription of small packages ...