Radiation therapy is generally painless, and side-effects generally increase with the dosage. Side-effects can occur during treatment, months or years after treatment, and after re-treatment.

During treatment the lining of the mouth, throat, esophagus, and bowel may be damaged by radiation. If the head and neck area is treated, temporary soreness and ulceration commonly occur in the mouth and throat. If severe, this can affect swallowing, and the patient may need painkillers and nutritional support (See liquid-diet for more information on liquid and soft diets) . The esophagus can also become sore if it is treated directly, or if, as commonly occurs, it receives a dose of collateral radiation during treatment of lung cancer.