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Chemotherapy and other drug therapies

Drug therapies work in different ways to destroy cancer cells, stop them from spreading or slow down their growth. Drugs used to treat cancer are also called anticancer drugs or anticancer agents. Drugs may also be used to lessen or relieve side effects of cancer or its treatment.

  • Chemotherapy (sometimes called chemo) destroys cancer cells or slows their growth. Some chemotherapy drugs are given on their own. But in most cases, several chemotherapy drugs are used together to destroy cancer cells.
  • Hormonal drug therapy slows the growth of cancers like breast, prostate and uterine cancer that use natural sex hormones (such as estrogen, progesterone and testosterone) to grow. Hormonal therapy stops the cancer cells from using the hormones they need to grow or prevents the body from making the hormone that is causing the cancer to grow.
  • Immunotherapy helps to strengthen or restore the immune system’s ability to fight cancer.
  • Targeted therapy uses drugs to target specific molecules (for example, genes or proteins) in cancer cells to stop them from growing and spreading. Targeting cancer cells means that these drugs don’t damage as many normal cells.
  • Supportive drugs prevent, manage or relieve side effects from cancer or cancer treatments. For example, bisphosphonates help strengthen bones and protect them against the effects of some cancers and their treatments.

Sometimes drug therapy is the only treatment you get. You may need only one type of drug therapy, or you may receive a combination of drug therapies.

Most drug therapies that treat cancer are systemic. This means that the drugs travel through the bloodstream and can attack cancer cells almost anywhere in the body.

Goals of drug therapy

As part of your treatment plan, drug therapy may be used in the following ways.

  • Drug therapy may help cure cancer by destroying all the cancer cells and lowering the chances that the cancer will come back.
  • It can try to control cancer by stopping cancer cells from growing and spreading or destroying cancer cells that have spread to other parts of the body.
  • Drugs can help prevent the growth or spread of new cancer cells.
  • Drugs can help relieve symptoms of cancer and help you feel more comfortable when curing cancer is not possible.

How and when drug therapy is used

Drug therapy may also be used alone to treat cancer or with other treatments.

Drug therapies may be used as adjuvant therapy. This means that drug therapies are given in addition to the first-line therapy. The drugs aim to destroy any cancer cells that are still in the body after surgery or radiation therapy and help reduce the risk that the cancer will come back (recur).

Sometimes drugs are given as neoadjuvant therapy. They are used to shrink a tumour before the first-line therapy. Shrinking the tumour makes it easier to remove with surgery or to treat with radiation.

Drug therapy given at the same time as other treatments is called concurrent therapy. For example, certain chemotherapy drugs may be given during the same period of time as radiation therapy. This may help make the cancer cells more sensitive to radiation.

Drug therapy may be used to control cancer if it is not possible to cure it.  The first drug or combination of drugs used to control cancer are called first-line, which may be followed later by further treatments called second-line.

In selected cases, drug therapy alone can be used to attempt to cure cancer, and in that situation it is called definitive or primary therapy.

Deciding which drugs to use

Chemotherapy and other drugs used to treat cancer are prescribed by a medical oncologist. This doctor will discuss with you the chemotherapy drugs that are most likely to work and decide the dose, schedule and length of treatment. Chemotherapy and other drug treatment plans, called regimens, depend on:

  • the type of cancer
  • the stage of the cancer
  • your age
  • your overall health and if there are other health problems (such as heart, liver or kidney diseases)
  • the types of cancer treatment you’ve already had or your planned treatment
  • your preference


Chemotherapy and other drugs used to treat cancer may not kill all the cancer cells the first time they are given. Some cancer cells may survive and continue to grow. Chemotherapy and other drugs are given according to a plan or schedule designed to kill as many cancer cells as possible. Chemotherapy and other drug treatment is often repeated and is usually given at regular intervals called cycles. Each cycle of these drugs is usually followed by a rest period (such as several days or weeks) when no treatment is given. The rest period allows normal cells to recover.


The length, timing and number of cycles of chemotherapy or other drugs depends on the combination of drugs used and the type of cancer. The medical oncologist usually sets the schedule when treatment starts. The schedule may also be decided later, based on how well the drug is working and how the body tolerates it.

Chemotherapy is given for about 3 to 6 months when the goal is to cure the cancer. Chemotherapy may be given for longer when it is used to relieve or control symptoms of advanced cancer (called palliative chemotherapy). Hormonal drug therapy or targeted therapy may be given for longer periods of time.

Calculating the dose

Doctors calculate doses of drugs in different ways, depending on the drug or drugs. Overall dose is usually based on the person’s body surface area (BSA), which is calculated using height and weight and expressed in metres squared. For a few drugs, the dose is based only on the person’s weight in kilograms.

The bodies of children and some adults process drugs differently, and they may have different levels of sensitivity. After taking the BSA into account, the doctor may change dosages for children and some adults. Dosages of some drugs may be adjusted for people who:

  • are elderly
  • don’t get the recommended amount of calories and nutrients
  • are obese
  • are taking or have taken other medicines
  • are receiving or have already received radiation therapy
  • have low blood cell counts
  • have liver or kidney disease

Chemotherapy and other drugs used to treat cancer are given at high enough doses to destroy the cancer cells and cure or control the cancer, while limiting side effects as much as possible.

Dose limitations

Very high doses of drugs can cause severe side effects, which may limit the amount of drug that can be given (called a dose-limiting factor). Sometimes the doctor will adjust the treatment to give you time to recover from side effects.

Maximum dose

Chemotherapy may be used again if cancer recurs (comes back). But for some drugs, there is a limit to how much can be given (called the maximum lifetime dose) because they can affect organs, such as the heart or lungs. If you have had the maximum lifetime dose, the drug cannot be used again.

How drugs are given

Most drugs used to treat cancer are given by an injection into a vein (intravenously, IV) or are taken by mouth (orally). Some drugs are given by an injection into an artery or certain parts of the body, such as a body cavity or into the fluid surrounding the spinal cord.


A substance that regulates specific body functions, such as metabolism, growth and reproduction.

Natural hormones are produced by glands. Artificial or synthetic hormones can be made in the lab.

immune system

The complex group of cells and organs that defend the body against infection, disease and foreign substances.

medical oncologist

A doctor who specializes in diagnosing and treating cancers using chemotherapy, biological therapy, hormonal therapy and supportive therapy.

The medical oncologist is often the primary healthcare provider for a person with cancer and will coordinate with other specialists.


A description of the extent of cancer in the body, including the size of the tumour, whether there are cancer cells in the lymph nodes and whether the disease has spread from its original site to other parts of the body.

Stages are based on specific criteria for each type of cancer.

The process of determining the extent of cancer in the body based on exams and tests is called staging.