Biological drugs often cause high costs for the healthcare system. When the patent of such a biological drug ("reference product") expires, similar drugs ("biosimilars") can enter the market as alternatives. This is a potential cost reduction for the healthcare system, which is still limited by high production costs of biosimilars. We simulate the market dynamics after one more more biosimilars enter the market, using a system dynamics model. Depending on different circumstances, a biosimilar can become market leader, reach significant market shares, or languish on a low level. Results include number of sales, revenue, and market shares for each company, as well as total costs for the healthcare system. The model can be used for negotiations between the healthcare system and industry, and can also support strategic planning.