Background <p>Dietary supplement use is common among patients with cancer, yet limited data exist on how patients perceive and use these products during treatment. Traditional research methods may not fully capture patient-driven motivations and experiences. Social media platforms such as Reddit provide a unique opportunity to examine real-world, unfiltered discussions about supplement use in the oncology setting.</p> Methods <p>Posts from the subreddit r/cancer published between November 2010 and December 2022 that referenced dietary supplement use were identified using natural language processing (NLP) techniques. A 10% random sample (<i>n</i> = 155) of the identified posts (<i>n</i> = 1,488) was analyzed qualitatively using directed content analysis. Posts were categorized by purpose (seeking vs. reporting use), intended use (treatment of cancer vs. symptom management), and supplement type (vitamin, mineral, other). Frequencies of themes related to supplement use, treatment goals, and decision-making motivations were assessed.</p> Results <p>Among users who specified their cancer type, breast, lung, and colorectal cancers were most frequently mentioned. 59% of posts were inquiries about supplement use, and 41% described personal experiences. Intended use was evenly divided between symptom management (51%) and cancer treatment (49%). Vitamins were the most commonly mentioned supplement category (38%). Reported motivations included managing fatigue, nausea, pain, and emotional distress. Discussions frequently reflected attempts to identify supportive strategies outside of conventional medical care.</p> Conclusions <p>Reddit represents a valuable, underutilized resource for understanding cancer patients’ perspectives on dietary supplement use. Insights derived from these online discussions highlight the importance of addressing supplement use in clinical encounters and underscore the need for further research to guide safe and effective integration of supplements into oncology care.</p>