Chemotherapy Drugs: How They Work

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Planning chemotherapy treatments

Most chemotherapy (chemo) drugs are strong medicines that have a fairly narrow dose range for safety and effectiveness. Taking too little of a drug will not treat the cancer well and taking too much may cause life-threatening side effects. For this reason, doctors must calculate chemo doses very precisely.

Determining chemo doses

Depending on the drug(s) to be given, there are different ways to determine chemo doses. Most chemo drugs are measured in milligrams (mg).

The overall dose may be based on a person’s body weight in kilograms (1 kilogram is 2.2 pounds). For instance, if the standard dose of a drug is 10 milligrams per kilogram (10 mg/kg), a person weighing 110 pounds (50 kilograms) would get 500 mg (10 mg/kg x 50 kg).

Some chemo doses are determined based on body surface area (BSA), which doctors calculate using height and weight. BSA is expressed in meters squared (m2).

Dosages for children and adults differ, even after BSA is taken into account. This is because children’s bodies process drugs differently. They may have different levels of sensitivity to the drugs, too. For the same reasons, dosages of some drugs may also be adjusted for people who:

  • Are elderly
  • Have poor nutritional status
  • Are obese
  • Have already taken or are currently taking other medicines
  • Have already had or are currently getting radiation therapy
  • Have low blood cell counts
  • Have liver or kidney diseases

Setting a chemo schedule (cycles)

Chemo is commonly given at regular intervals called cycles. A cycle may involve a dose of one or more drugs followed by several days or weeks without treatment. This gives normal cells time to recover from drug side effects. Sometimes, doses may be given a certain number of days in a row, or every other day for several days, followed by a period of rest. Some drugs work best when given continuously over a set number of days.

Each drug is given on a schedule that’s carefully set up to make the most of its anti-cancer actions and minimize side effects. If more than one drug is used, the treatment plan will say how often and exactly when each drug should be given. The number of cycles given may be decided before treatment starts, based on the type and stage of cancer. In some cases, the number is flexible, and will take into account how the treatment affects the cancer and the person’s overall health.

Changes in doses and schedules

In most cases, the most effective doses and schedules of drugs to treat specific cancers have been found by testing them in clinical trials. It’s important, when possible, to get the full course of chemo, the full dose, and keep the cycles on schedule. This will gives the best chance of getting the maximum benefit from treatment.

There may be times, though, when serious side effects require doctors to adjust the chemo plan (dose and/or schedule) to allow the body time to recover. In some cases, supportive medicines such as growth factors may be used to help the body recover more quickly. Again, the key is to give enough medicine to kill the cancer cells without causing other serious problems.

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