The side-effects of radiation depend on the treatment dose and the part of the body that is treated. During radiation therapy, people are likely to become extremely tired, especially in the later weeks of treatment. Rest is important, but doctors usually advise patients to try to stay as active as they can. Radiation therapy (or radiotherapy) techniques have steadily improved over several decades. Treatments now target the cancers more precisely, and more is known about selecting radiation doses. These advances are expected to reduce the number of secondary cancers resulting from radiation therapy. However, oncologists prescribe radiation therapy only when its benefits (improved survival rate and relief of symptoms) outweigh the risk of this and other complications.

It is common to lose hair in the treated area and for the skin to become red, dry, tender, and itchy. There may also be permanent darkening or "bronzing" of the skin in the treated area.

When the chest and neck are treated, patients may have a dry, sore throat and some trouble swallowing. Sometimes, they have shortness of breath or a dry cough. Radiation therapy to the abdomen may cause nausea, vomiting, diarrhea, or urinary discomfort. Often, changes in diet or medicine can ease these problems.

Radiation therapy also may cause a decrease in the number of white blood cells or platelets. If that happens, patients need to be careful to avoid possible sources of infection or injury. The doctor monitors a patient's blood count very carefully during radiation treatment. If necessary, treatment may have to be postponed to let the blood counts return to normal.

Other possible side-effects include headaches and cognitive problems (if treating metastatic cancer of the brain), and lung and respiratory problems (if treating cancer that has spread to the lungs or bones in the chest).

Although the side-effects of radiation therapy can be difficult to cope with, they can usually be treated or controlled. It may also help to know that, in most cases, side-effects are not permanent. However, patients may want to discuss with their doctor the possible long-term effects of radiation treatment on their ability to have children, and the increased chance of secondary cancers after treatment is over. Loss of fertility may be temporary or permanent, depending on the patient's age and on whether the testes or ovaries received radiation. For men, "sperm banking" before treatment may be a choice. Women's menstrual periods may stop, and they may have hot flashes and vaginal dryness. Menstrual periods are more likely to return for younger women.