Cancer and its treatments may cause weight loss, weight gain, and weight fluctuations. Some of the weight changes come from amped-up hunger or suppressed appetite from the cancer‘s direct impact and from medications. If you have metastatic breast cancer (MBC), some medications seem to especially instigate weight gain. Weight loss can be beneficial if you were overweight or diagnosed with obesity prior to treatment or if you gain weight after treatment. Obesity can drive cancer, so weight gain should be addressed.