OBJECTIVES: Copays (or copayments) for health-technologies, such as pharmaceutical products, are used by governments and insurers to prevent moral hazard, reduce unnecessary utilization of resources, and contribute to the cost of healthcare. Nevertheless they may also reduce access to necessary care, and the financial protection associated with health insurance. In addition, from the perspective of the manufacturer, the introduction of copays may determine unexpected and abrupt falls in demand. The methodology here presented uses data extracted from surveys on product utilization and patient income, prior and after the introduction of copays in various countries and regions, to predict the potential impact of copays on the demand for a pharmaceutical product in a given country or region. METHODS: This approach uses multi-level multivariate regressions and a micro-economic model of demand (which assumes maximization of utility and preference independence) to anticipate changes in utilization (and therefore in demand) as a function of the amount of copay charges. Data from a computer simulation were used to test the method. RESULTS: A non-linear relationship between copay and drug-consumption, determined by the combined effect of the amount charged and of the average population income was identified, predicting changes in aggregate demand and in income-specific demand. For instance: if we consider two countries both adopting a 1 Euro co-pay charge on a (not easily substitutable) product and whose average income differs by 20%, we expect the  demand to be almost 3% lower in the country with the lower income than in the country with the higher income. CONCLUSIONS: Under general market assumptions, it is possible to build models that estimate the potential impact of copay charges. These models can be used to help design health policies and market strategies.