There are several situations in which chemotherapy may be used to treat colon or rectal cancers.

Adjuvant chemotherapy: The use of chemotherapy after surgery, known as adjuvant chemotherapy, can increase the survival rate for patients with some stages of colon cancer and rectal cancer. It is given when there is no evidence of cancer but there is a chance that it might come back. The theory behind adjuvant therapy is that a small number of cancer cells may not have been removed by surgery or may have escaped from the primary tumor and settled in other parts of the body. The hope is that the chemotherapy can kill these cells, wherever they may be.

Neoadjuvant chemotherapy: For some rectal cancers, chemotherapy is given (along with radiation) before surgery to try to shrink the cancer and make surgery easier. This is known as neoadjuvant treatment.

Chemotherapy for advanced cancers: Chemotherapy can also be used to help shrink tumors and relieve symptoms for more advanced cancers. While it is very unlikely to be curative in such situations, it may greatly extend survival time in some people.

Drugs used to treat colorectal cancer

Several drugs can be used to treat colorectal cancer. Often, 2 or more of these drugs are combined to try to make them more effective.

Fluorouracil (5-FU): This drug had been around for several decades, and it is part of most chemotherapy regimens for colorectal cancer. It is often given together with another drug called leucovorin (or folinic acid), which increases its effectiveness.

There are several different schedules for using this drug. It may be given as an infusion over 2 hours, or (more commonly) as a quick injection followed by continuous infusion over 1 or 2 days. For continuous infusions, the patient wears a small battery-operated pump that infuses 5-FU into an intravenous (IV) catheter.

For most chemotherapy regimens, treatment with 5-FU is repeated every 2 weeks, over a period of 6 months to a year.

The possible side effects of this drug include nausea, loss of appetite, mouth sores, diarrhea, low blood cell counts, sensitivity to sunlight, and a syndrome of hand and foot redness that is sometimes accompanied by blistering or skin peeling.

Capecitabine (Xeloda): This is a chemotherapy drug in pill form. It is usually taken twice a day for 2 weeks, followed by a week off. Once in the body, it is changed to 5-FU when it gets to the tumor site. This drug seems to be about as effective as giving continuous intravenous 5-FU.

While this drug may be taken at home as a pill, it is still a strong chemotherapy medicine. The possible side effects that are similar to those listed for 5-FU. Although most of the side effects seem to be less common with this drug than with 5-FU, problems with the hands and feet are more common.

Irinotecan (Camptosar): This drug is often combined with 5-FU and leucovorin (known as the FOLFIRI regimen) as a first-line treatment for advanced colorectal cancer. In some cases it may be tried by itself as a second-line treatment if other chemotherapy drugs are no longer effective. It is given as an IV infusion over 30 minutes to 2 hours.

One problem with irinotecan is that some people are unable to break down the drug so it stays in the body and causes severe side effects. This is due to an inherited genetic variation that can be tested for. The simplest test is to measure the blood level of bilirubin, a substance made in the liver. If it is slightly elevated, this can be a sign of the genetic variation that makes people sensitive to irinotecan. So far, most doctors aren't routinely testing for the genetic variant itself.

The major possible side effects of irinotecan are severe diarrhea and low blood counts, although other effects such as nausea are possible as well. Your doctor will likely give you medicine to take before treatment to help prevent diarrhea. You need to tell your doctor right away if you develop diarrhea or any other side effects. Your doctor may not advise irinotecan if you are elderly or have serious health problems. In rare cases, severe side effects can even be fatal.

Oxaliplatin (Eloxatin): This drug is usually combined with 5-FU and leucovorin (known as the FOLFOX regimen) or with capecitabine (known as the CapeOX regimen) as a first- or second-line treatment for advanced colorectal cancer. It may also be used as adjuvant therapy after surgery for earlier stage cancers. Oxaliplatin is given as an IV infusion over 2 hours, usually once every 2 or 3 weeks.

Oxaliplatin can affect peripheral nerves, which can cause numbness, tingling, and intense sensitivity to temperature in the extremities, especially the hands and feet. This goes away after treatment has stopped in most patients, but in some cases it can cause long-lasting nerve damage. If you are going to be getting oxaliplatin, talk with your doctor about side effects beforehand, and let him or her know as soon as you develop numbness and tingling or other side effects.

Side effects of chemotherapy

Chemotherapy drugs work by attacking cells that are dividing quickly, which is why they work against cancer cells. But other cells in the body, such as those in the bone marrow, the lining of the mouth and intestines, and the hair follicles, also divide quickly. These cells are also likely to be affected by chemotherapy, which can lead to side effects.

The side effects of chemotherapy depend on the type and dose of drugs given and the length of time they are taken. General side effects of chemotherapy drugs can include:

• hair loss 
• mouth sores 
• loss of appetite 
• nausea and vomiting 
• increased chance of infections (due to low white blood cell counts) 
• easy bruising or bleeding (due to low blood platelet counts) 
• fatigue (due to low red blood cell counts)

Along with these, some side effects are specific to certain medicines. These are discussed above in the descriptions of the individual drugs.

Most side effects are short-term and tend to go away after treatment is finished. There are often ways to lessen these side effects. For example, drugs can be given to help prevent or reduce nausea and vomiting. Do not hesitate to discuss any questions about side effects with the cancer care team.

You should report any side effects or changes you notice while getting chemotherapy to your medical team so that they can be treated promptly. In some cases, the doses of the chemotherapy drugs may need to be reduced or treatment may need to be delayed or stopped to prevent the effects from getting worse.

Elderly people seem to be able to tolerate chemotherapy for colorectal cancer fairly well. There is no reason to withhold treatment in otherwise healthy people simply because of age.

For more general information about chemotherapy, please see the separate American Cancer Society document, Understanding Chemotherapy: A Guide for Patients and Families.

Targeted therapies

As researchers have learned more about the gene and protein changes in cells that cause cancer, they have been able develop newer drugs that specifically target these changes. These targeted drugs work differently than standard chemotherapy drugs. They often have different (and less severe) side effects. At this time, they are most often used either along with chemotherapy or by themselves if chemotherapy is no longer working.

Bevacizumab (Avastin): Bevacizumab is a manmade version of an immune system protein called a monoclonal antibody. This antibody targets vascular endothelial growth factor (VEGF), a protein that helps tumors form new blood vessels to get nutrients (a process known as angiogenesis). Bevacizumab is most often used along with chemotherapy drugs as a first- or second-line treatment for metastatic colorectal cancer.

Bevacizumab is given by intravenous (IV) infusion, usually once every 2 or 3 weeks. While it has been shown to help improve survival when added to chemotherapy, it can also add to the side effects. Rare but possibly serious side effects include blood clots, holes forming in the colon (requiring surgery to correct), heart problems, and slow wound healing. More common side effects include high blood pressure, tiredness, bleeding, low white blood cell counts, headaches, mouth sores, loss of appetite, and diarrhea.

Cetuximab (Erbitux): This is a monoclonal antibody that specifically attacks the epidermal growth factor receptor (EGFR), a molecule that often appears in high amounts on the surface of cancer cells and helps them grow.

Cetuximab is used in metastatic colorectal cancer, usually after other treatments have been tried. It can be used either with irinotecan or by itself in those who can't take irinotecan or whose cancer is no longer responding to it.

Cetuximab is given by IV infusion, usually once a week. A rare but serious side effect of cetuximab is an allergic reaction during the first infusion, which could cause problems with breathing and low blood pressure. You may be given medicine before treatment to help prevent this. Other, less serious side effects may include an acne-like rash, headache, tiredness, fever, and diarrhea.

Panitumumab (Vectibix): Panitumumab is another monoclonal antibody that attacks colorectal cancer cells. Like cetuximab, it targets the EGFR protein. It is used by itself to treat metastatic colorectal cancer after other treatments have been tried.

Panitumumab is given by IV infusion, usually once every 2 weeks. Most people develop skin problems such as a rash during treatment, which in some cases can lead to infections. Other possible serious side effects are lung scarring and allergic reactions to the drug. Sensitivity to sunlight, fatigue, diarrhea, and changes in fingernails and toenails are also possible.