A side effect is defined as a problem (or problems) that may occur as a result of a prescribed treatment. Such problems are effects that go beyond that which was intended, or are conditions that arise as a result.
Most cancer treatments have side effects, such as hair loss, nausea and loss of appetite, fatigue, lesions and more. The reason is because traditional treatments designed to kill cancerous cells will usually affect healthy tissue as well.
One reason cancers are difficult to treat lies in the fact that cancer cells do not in fact constitute a "foreign" pathogen, or invader. Cancer in the human body is actually similar to what happens to a geographic area when the population grows too large for available space and resources, or in an economic system that has become too top-heavy and monopolistic.
Healthy cells are programmed to live, function and eventually, age and die like all living things. This "cell death" is called apoptosis. When these cells fail to die off and continue to grow and multiply in an uncontrolled manner, it is known as cancer.
Traditional treatments for cancer include chemotherapy and radiation therapy, which is similar to using toxic pesticides and fire in order to clear weeds out of a garden. These methods will kill off noxious weeds, but they will surely kill desirable plants as well.
It is the same with chemotherapy and radiation. Invariably, some healthy tissue is harmed or killed in the process, and the overall procedures will lead to toxic conditions in the body that causes the side effects outlined above. In fact, chemotherapy drugs, when spilled in a hospital, must be treated as toxic waste; specialists in HAZMAT suits are required for cleanup. Of course, the effects of nuclear radiation on the human body are well documented. This is why every attempt is made to focus the treatment as tightly as possible around the affected area.
Recent medical studies indicate that cancers thrive in an acidic, low-oxygen environment. This knowledge has led to some alternative treatments that may eventually eliminate the need for more toxic treatments.