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 Immunotherapy for Cancer Immunotherapy is a cancer treatment that uses your own immune system to fight your cancer. Your immune system consists of cells, tissues, and organs that protect your body from illness and infection... read more is different.

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:

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:

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Postprandial Hypotension
Postprandial hypotension is an excessive decline in blood pressure after a meal. When it occurs, the person may feel lightheaded or dizzy, and is at risk of falling. For which of the following groups is the risk of postprandial hypotension highest?
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