Students in higher education have been shown to have difficulties in developing their critical thinking skills, such as analysis and problem solving, reasoning and argumentation. Open-ended tasks offer opportunities for students to develop their own interpretations of various sources, to critically analyse domain-specific knowledge and utilize that knowledge in their argumentation. This study focuses on the ability of new Master's students (n=37) to utilize pharmaceutical knowledge from different sources in producing written arguments and counter-arguments in the context of open-ended assignment task. The data were analysed by qualitative content analysis. The results showed that there was substantial variation in how students analysed and processed pharmaceutical knowledge as well as how they utilized that knowledge in their argumentation. While some students were able to provide comprehensive analysis of the different sources, others superficially analysed and processed the sources and struggled to generate convincing arguments. Students' written responses were typically one-sided: only a few students provided counter-arguments associated with the pharmaceutical problem-solving situation presented in the task. Understanding the nature of the challenges in argumentation and knowledge processing encountered by pharmacy students can help pharmacy educators to modify their pedagogical practices to better support students' learning. [The page range (37-53) noted on the .pdf is incorrect. The correct page range is p37-51.]