Chemotherapy Drugs: How They Work

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Deciding which chemotherapy drugs to use

In some cases, the best choice of doses and schedules for giving each chemotherapy (chemo) drug are clear, and most doctors would recommend the same treatment. In other cases, less may be known about the single best way to treat people with certain types and stages of cancer. In these cases, different doctors might choose different drug combinations with different schedules.

Factors to consider in choosing which drugs to use include:

  • The type of cancer
  • The stage of the cancer (how far it has spread)
  • The patient’s age
  • The patient’s overall health
  • Other serious health problems (such as heart, liver, or kidney diseases)
  • Types of cancer treatments given in the past

Doctors take these factors into account, along with information published in medical journals and textbooks describing the outcomes of similar patients treated with chemo.

Chemotherapy regimens or treatment plans may use a single drug or a combination of drugs. Most people with cancer get more than one chemo drug. This is typically more effective than a single drug, because the cancer cells can be attacked in several different ways. Doctors must also consider side effects of each drug and potential interactions among the drugs.

Considering the side effects

Different drugs have different side effects. It’s often better to use moderate doses of 2 drugs that will cause bearable side effects instead of very high doses of one drug that might cause severe side effects and maybe permanently damage an organ. But there are exceptions to this rule, and a single chemotherapy drug may be the best option for some people with certain types of cancer.

Doctors try to give chemo at levels high enough to cure or control the cancer, while keeping side effects at a minimum. They also try to avoid multiple drugs that have similar side effects.

For more on chemo side effects and how to deal with them, please visit the “Treatment and Side Effects” section of our website.

Avoiding drug interactions

When looking at how to best combine 2 or more chemo drugs, doctors must also consider potential interactions between the drugs. They have to look at interactions between chemo drugs and other medicines the person is taking, too, including vitamins and non-prescription medicines. In some patients, these interactions may make side effects worse. In others, they may interfere with how well the chemo works.

It’s important that you tell your doctor about all medicines you are taking, including vitamins, herbal or dietary supplements, and non-prescription medicines – even if you only take them “as needed.”

For instance, platelets are the blood cells that cause blood to clot and prevent bleeding. Many chemo drugs slow down the bone marrow’s production of platelets for a time. Taking aspirin or other related drugs can also weaken blood platelets. This isn’t a problem for healthy people with normal platelet counts. But if a person has low platelet counts from chemo, this combination may put them at risk of a serious bleeding problem.

How vitamins affect chemo drugs

Many people want to take an active role in improving their overall health. They want to help their body’s natural defenses fight the cancer and speed up their recovery from chemo.

Because most people think of vitamins as a safe way to improve health, it’s not surprising that many people with cancer take high doses of one or more vitamins. But few know that some vitamins might make their chemo less effective.

Certain vitamins, such as A, E, and C act as antioxidants. This means that they can prevent formation of ions (free radicals) that damage DNA. This damage is thought to have an important role in causing cancer. But some chemotherapy drugs (as well as radiation treatments) work by producing these same types of free radical ions. These ions severely damage the DNA of cancer cells so the cells are unable to grow and reproduce. Some scientists believe that taking high doses of antioxidants during treatment may make chemo or radiation less effective.

Few studies have been done to thoroughly test this theory. But until we know more about the effects of vitamins on chemo, many doctors recommend the following during chemo treatment:

  • If your doctor has not told you to take vitamins, it’s best not to take any.
  • A simple multivitamin is probably OK for people who want to take a vitamin supplement, but always check with your doctor first.
  • It’s safest to avoid taking high doses of antioxidant vitamins or supplements during cancer treatment. Ask your doctors if and when it might be OK to start such vitamins after treatment.
  • If you are concerned about nutrition, you can usually get plenty of vitamins by eating a well-balanced diet.

Visit “Nutrition for People With Cancer” on our website to learn more about the importance of good nutrition during and after cancer treatment.

Last Medical Review: 02/06/2015
Last Revised: 02/06/2015