Chemotherapy — the use of medications to treat cancer — has played a major role in cancer treatment for half a century. Years of testing and research have proved chemotherapy to be an effective cancer treatment. It may be your only treatment, or it may be used in combination with other treatments, such as surgery and radiation therapy.

Chemotherapy works by killing rapidly dividing cells. These cells include cancer cells, which continuously divide to form more cells, and healthy cells that divide quickly, such as those in your bone marrow, gastrointestinal tract, reproductive system and hair follicles. Healthy cells usually recover shortly after chemotherapy is complete, so for example, your hair starts growing again.

If your doctor recommends chemotherapy to you, you may feel anxious. But by becoming informed about chemotherapy — what it is, why and how it's used, and what you can expect — you may feel more comfortable with the treatment process.

One of chemotherapy's main advantages is that — unlike radiation, which treats only the area of the body exposed to the radiation — chemotherapy treats the entire body. As a result, any cells that may have escaped from the original cancer are treated.

Depending on what type of cancer you have and whether it has spread, your doctor may use chemotherapy to:

• Eliminate all cancer cells in your body, even when cancer is widespread
• Prolong your life by controlling cancer growth and spread
• Relieve symptoms and enhance your quality of life

In some cases, chemotherapy may be the only treatment you need. More often, it's used in conjunction with other treatments, such as surgery, radiation or a bone marrow transplant, to improve results. For example, you may receive:

• Neoadjuvant therapy. The goal of neoadjuvant therapy is to reduce the size of a tumor before surgery or radiation therapy.
• Adjuvant therapy. Given after surgery or radiation, the goal of adjuvant therapy is to eliminate any cancer cells that might linger in your body after earlier treatments.

Chemotherapy may not be limited to a single drug. Most chemotherapy is given as a combination of drugs that work together to kill cancer cells. Combining drugs that have different actions at the cellular level may help destroy a greater number of cancer cells and might reduce your risk of cancer developing resistance to one particular drug. Your doctor will recommend drug combinations that have been tested in people with similar conditions and have been shown to have some effect against your particular type of cancer.

What chemicals your doctor recommends is generally based on the type, stage and grade of your cancer, as well as your age, general health and your willingness to tolerate certain temporary side effects. Some types of chemotherapy medications commonly used to treat cancer include:

• Alkylating agents. These medications interfere with the growth of cancer cells by blocking the replication of DNA.
• Antimetabolites. These drugs block the enzymes needed by cancer cells to live and grow.
• Anti-tumor antibiotics. These antibiotics — different from those used to treat bacterial infections — interfere with DNA, blocking certain enzymes and cell division and changing cell membranes.
• Mitotic inhibitors. These drugs inhibit cell division or hinder certain enzymes necessary in the cell reproduction process.
• Nitrosoureas. These medications impede enzymes that repair DNA.

You usually receive chemotherapy in cycles, depending on your condition and which drugs are used. Cycles may include taking the drugs daily, weekly or monthly for a few months or several months, with a recovery period after each treatment. Recovery periods allow time for your body to rest and produce new, healthy cells.

Chemotherapy drugs can be taken in a number of forms. Your doctor determines what form(s) to use primarily based on what type of cancer you have and what drug(s) will best treat your cancer. Examples of different forms of chemotherapy include:

• Intravenous (IV). Chemotherapy is injected into a vein, using a needle inserted through your skin. This allows rapid distribution of the chemotherapy throughout your entire body.
• Oral. You swallow this form of chemotherapy as a pill.
• Topical. This type of drug is applied to your skin to treat localized skin cancers.
• Injection. Using a needle, your doctor injects the drug directly into a muscle, under your skin or into a cancerous area on your skin.

Chemotherapy medications, regardless of how they're given, generally travel in your bloodstream and throughout your entire body. The intravenous route is the most common, allowing chemotherapy drugs to spread quickly through your system. In cases in which your doctor wants to direct chemotherapy to a more confined area — for example, to ensure a tumor is exposed to more of the drug — he or she may insert a catheter directly into that area or into a blood vessel supplying the tumor.

Because chemotherapy drugs can affect healthy cells, one of their disadvantages is that you may experience side effects, some temporary and some longer term. Not every drug will cause every side effect. Your doctor can tell you what to expect from the drugs you're receiving.

Temporary side effects might include:

• Hair loss
• Dry mouth
• Mouth sores (stomatitis)
• Difficult or painful swallowing (esophagitis)
• Nausea
• Vomiting
• Diarrhea
• Constipation
• Fatigue
• Bleeding
• Susceptibility to infection
• Infertility
• Loss of appetite
• Changes in the way food tastes
• Cognitive impairment, sometimes referred to as chemo brain
• Liver damage

How long these temporary side effects last depends on what drug(s) you're taking and for how long. Most side effects will subside shortly after you stop your chemotherapy treatments. And most short-term side effects can be minimized with medication. For example, your doctor can give you medications to help relieve nausea or build up your blood counts. If side effects make you uncomfortable, tell your doctor. If you find that the side effects are more than you're willing to endure, you can change treatments.

As people with cancer are living longer after treatment, doctors have discovered that some treatments cause long-lasting side effects or side effects that become apparent long after treatment ends. These long-term side effects are rare. Before you begin treatment, discuss with your doctor what long-term effects you might experience. Some chemotherapy drugs can cause:

• Organ damage, including problems with your heart, lungs and kidneys
• Nerve damage
• Blood in your urine (hemorrhagic cystitis)
• Another cancer, including Hodgkin's disease and non-Hodgkin's lymphoma, leukemia and some tumors

Your doctor can tell you what signs and symptoms to watch for after treatment. Knowing what long-term side effects to watch for can help you stay healthy after treatment.

While beginning chemotherapy can be frightening, know that new medications are helping reduce unpleasant side effects. But chemotherapy will always cause some significant side effects. Keep in mind that many people with cancer are living longer than ever — thanks partly to chemotherapy.