In this study we exploit a regulation in Colombia that exogenously changes health insurance coverage of young adult dependents, specifically those turning 18 years old, to analyze the effects of losing health insurance coverage on their health service usage and health status. We assess this effect using a regression discontinuity design (RDD) and data from the Encuesta Nacional de Calidad de Vida Survey for Colombia from 2010 to 2013. Losing coverage implies an increase in the cost of some medical services which may reduce their consumption (i.e. preventive services). Additionally, since under Colombian regulations, emergency department (ED) visits cannot be denied to anyone, regardless of health insurance status, uninsured young adults tend to use this service more instead of regular medical services (such as preventive healthcare or visits to physicians or specialists). We find, consistent with the change in relative prices, that losing health insurance when turning 18 years old increases the visits to the ED, reduces preventive care visits with a physician, and reduces the usage of other medical services. These results imply a substitution of cheaper medical services for more expensive ones when individuals turn 18 years old in Colombia.