If you have cancer, depression is much more than just a dark mood. The illness, which strikes about up to 25 percent of all cancer patients (compared with about 7 percent of the general public), can sap your immune system, and weaken your body’s ability to cope with the disease.

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How does depression affect cancer patients?

Patients fighting both depression and cancer feel distressed, tend to have trouble with everyday tasks, and often can’t follow medical advice. Indeed, doctors believe that depression, if left untreated, can shorten a cancer patient’s life.

A study by the California Department of Health and Human Services found that cancer patients without strong social ties were three more times likely to die earlier than their socially active counterparts.

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Can treating depression help?

Yes, according to researchers. When a person suffers from cancer and depression, treatment for the mind can give the rest of the body a huge boost.

A study of women with advanced breast cancer, conducted at Stanford University, found that those who attended weekly support groups lived an average of 18 months longer than those who didn’t. Although more recent research has not found such an effect on survival rates, it has shown that support groups improve the quality of life for patients.

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A later study at UCLA of patients with malignant melanoma found an equally remarkable trend. Patients who participated in group therapy were three times more likely to be alive five to six years later than those who didn’t receive therapy.

Antidepressants may also play an important role in the fight against cancer. An Israeli study found that antidepressants increased the levels of natural killer cells — soldiers of the immune system that destroy cancer cells and other intruders — in a group of cancer patients.

The bottom line is that treating depression not only eases symptoms of pain, nausea, and fatigue, it may help you live longer and enjoy a better quality of life.

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Can depression actually increase the risk of cancer?

Since depression can hamper natural killer cells (lymphocytes that kill cancer cells and microbes) and