The use of Complementary Alternative Medicine (CAM) in patients with cancer is increasing. CAM is associated with potential toxicity and drug interactions, particularly with chemotherapy. Here, we report a case of cytolysis and hepatic cholestasis in a patient who was self-medicated with a mushroom powder-based alternative therapy containing Agaricus blazei Murril (ABM) during cancer treatment. A 43-year-old woman with metastatic colorectal cancer and hepatic metastases was admitted to our hospital for intravenous chemotherapy. Markers of hepatic grade 3 cytolysis and cholestasis were identified during the pretreatment consultation. The baseline results were within normal limits. The chemotherapy was immediately canceled, and further tests were performed. After the investigation, the patient reported taking three mushroom powder-based capsules per day since November 2023. The dietary supplement contained ABM and Hericium erinaceus (HE) powder. After Pharmaceutical analysis, treatment with the supplement was discontinued, and the patient has not resumed. The changes in liver function were also favorable. In our case, given the improvement in liver function after CAM discontinuation, hepatic cytolysis appeared to be linked to ABM consumption despite the patient's liver metastases. Pharmaceutical analysis of CAM is essential to ensure the safety and optimization of cancer treatments. Patients should also communicate their CAMs to healthcare professionals and be aware of the consequences of consuming these dietary supplements. Finally, collaboration between pharmaceutical teams and oncologists is essential for optimal management of cancer patients.