The American Cancer Society notes that fatigue is a common side effect of radiation treatment. Fatigue is defined as feeling tired all the time, even after resting. A number of factors can lead to fatigue, including low red blood cell count, pain, poor nutrition, stress and chemotherapy drugs.
According the American Society of Clinical Oncology, radiation side effects can occur in the head, neck, chest, stomach and pelvic regions. Dry mouth, sores on the gums, difficulty swallowing and swelling are head and neck issues. Some patients experience nausea, vomiting and diarrhea after radiation. Other serious side effects are shortness of breath, chronic cough, incontinence and rectal bleeding.