Merck Manual

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Quick Facts



The Manual's Editorial Staff

Last full review/revision Jul 2021| Content last modified Jul 2021
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What is chemotherapy?

Chemotherapy is a medicine that destroys cancer cells. Chemotherapy works by shutting down cell growth. But since all cells in the body grow, chemotherapy medicines also destroy some normal cells and cause side effects. Not all drugs that treat cancer are considered chemotherapy, for example, immunotherapy is different.

  • Chemotherapy doesn't work on all cancers

  • There are many different chemotherapy medicines—the one you get will depend on the type of cancer you have

  • Most chemotherapy drugs are given through your vein (IV)

  • Doctors may give you several chemotherapy medicines at the same time or may combine chemotherapy with another type of cancer treatment, such as surgery or radiation therapy

  • Doctors may give you other medicine to lessen the side effects of chemotherapy

Sometimes after you have gotten a chemotherapy drug for a while, the cancer cells become resistant to it. The chemotherapy stops killing them. If this happens, your doctor may try a different drug.

What are the common side effects of chemotherapy?

Chemotherapy has a reputation for unpleasant and sometimes dangerous side effects. New chemotherapy drugs are often less bothersome than older ones. And doctors now have better treatments for some side effects.

The most common side effects are:

  • Feeling sick to your stomach or throwing up

  • Feeling less hungry than usual

  • Weight loss

  • Feeling weak and tired

  • Diarrhea (frequent, loose, or watery poop)

  • Losing your hair

  • Mouth or nose sores

Chemotherapy often affects healthy blood-forming cells in the bone marrow. This can lower your number of blood cells, which can cause:

  • Low blood count (anemia), if your red blood cell count is lowered

  • Serious infections, if your white cell count is lowered

  • Bleeding, if your platelet count is lowered

Anemia can cause:

  • Weakness

  • Feeling dizzy

  • Trouble breathing or chest pain

Infections from low white cell count can cause:

  • Fever

Low platelet count can cause:

  • Bruising easily

  • Bleeding from your nose, gums, or rectum

Chemotherapy can also affect other organs besides your bone marrow and cause other medical problems:

  • Damage to your lungs, heart, or liver

  • Infertility (trouble getting pregnant)

  • Sometimes a higher chance of getting another cancer such as leukemia

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