Reishi mushroom (Ganoderma lucidum) ranked as the 4th most searched supplement on Memorial Sloan Kettering's About Herbs database in 2025, reflecting how widely cancer patients are self-prescribing it — yet 60.5% of women on adjuvant cancer treatment never discuss supplement use with their doctors. The evidence for reishi during radiation is limited to preclinical data, and its spore supplements have been associated with elevated CA72-4 tumor markers in GI cancer patients, which can lead clinicians to misread disease progression. Reishi's antioxidant properties also raise a specific concern during cancer treatment: high-dose antioxidant supplements can theoretically protect tumor cells from radiation-induced damage, and one randomized trial in head and neck cancer found antioxidant use reduced short-term side effects but increased local recurrence.