What are the side effects of treatments for stomach cancer?

Multiple: I am not sure what modalities you are referring to asking about side effects. If it is chemo, then side effects may be various and multiple - nausea/vomiting/hair loss/diarrhea/weakness/allergy reactions etc. Radiation treatment may produce scarring of the exposed tissues and long term effects resulting in the development of other types of cancer.

Multiple: There are many types of treatment, ranging from surgery to chemotherapy to radiation. In general, some of the most common side effects include nausea, vomiting, weight loss, poor energy level, sensation of early filling, and loss of appetite. Specific chemotherapy drugs might cause skin sensitivity, mouth sores, and numbness/tingling of the fingers/toes.