There has been a long-standing interest in how groups collaboratively solve problem and cross-fertilize knowledge assets. Knowledge co-creation is one of the hallmarks of innovation in organizations. Using a multiple case study approach, we analyze the use of social media by patients, their friends and families, caregivers, and organizations in the context of breast and prostate cancer to see how they collaborate to produce new knowledge. Based on our results, we propose a theoretical model that explains the process of knowledge co-creation. Three phases underlie this process: initiation, transition, and normalization. For each phase, we identify the key activities, drivers and challenges that are faced by the enabling actors. Our model accounts for the dynamic as well as the temporal aspect of knowledge co-creation in the context of cancer care, to provide a richer account of this phenomenon.