Alcohol and cancer aren’t always the best bedfellows. The answer is “it depends.” According to the American Cancer Society, alcohol can sometimes clash with chemotherapy, since the liver processes the chemotherapy meds. You might know that alcohol can cause liver inflammation, and this can interfere with the chemotherapy doing its job. Another potential issue is that mouth sores are a common side effect of chemo and radiation therapies, and alcohol (including the alcohol in mouthwash) can exacerbate those mouth sores.
You should never drink heavily, whether you’re in cancer treatment or not. As for moderate alcohol use (up to a drink a day for women, two for men, according to the National Cancer Institute), you’ll definitely want to ask your doctor, who can review the particulars of your cancer therapy and your cancer type, and give an informed opinion. If the treatment doesn’t clash with alcohol use, and your doctor gives the OK, then enjoy a drink on occasion—but keep it in moderation