Drug interactions are an important source of medical error and a topic of particular interest to patient audiences. Patients must be informed to be able to participate in decision-making affecting their health. This paper explores the availability of drug interaction checkers in Scandinavia and the prevalence and characteristics of usability issues preventing patients from benefiting from them. Drug interaction checkers were sampled and evaluated through heuristic evaluations. Issue-based data were analyzed through descriptive statistics, as well as single-case and cross-case qualitative analyses. The findings were interpreted side-by-side using a mixed-methods approach. The results showed a multitude of usability issues. Catastrophic issues indicating the safety of dangerous drug pairings were found in two of the checkers. Results also showed that the checkers lacked adaptive design, patient-oriented content, and adherence to basic design principles. A positive correlation was observed between system complexity and number of usability issues. We suggest that this comes from a lack of systematic design approach. The market for Scandinavian drug interaction checkers was as such characterized by a limited selection of checkers known to be used by patients for their utility, but failing to accommodate them in terms of system quality.