What chemotherapy can do is temporarily increase acid load in specific situations, which explains where the claim comes from. For example, rapid cancer cell breakdown can trigger Tumor lysis syndrome, releasing acidic substances into the blood, while treatment stress on organs or low oxygen use can contribute to Lactic acidosis. These are exceptions, not the norm, and are closely monitored in clinical care. The “proof” against the general claim is physiological: the body’s buffering systems (lungs, kidneys, blood chemistry) actively maintain stable pH, meaning chemotherapy cannot simply make the entire body chronically acidic under normal conditions.