Some drugs, especially those available to people without a prescription, have a fairly wide therapeutic index. This means that wide ranges of doses can be used effectively and safely. For example, the label on a bottle of aspirin may suggest taking 2 tablets for a mild headache. But one tablet (half the dose) is likely enough to help many people.

Most chemotherapy drugs, on the other hand, are strong medicines that have a fairly narrow range of safe and effective doses. Taking too little of a drug will not effectively treat the cancer and taking too much may cause life-threatening side effects. For this reason, doctors must calculate chemotherapy doses very precisely.

Doses

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

The overall dose is sometimes 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 50 kilograms (110 pounds) would receive 500 mg (50 kg x 10 mg/kg).

Some chemotherapy doses are determined based on body surface area (BSA), which doctors calculate using your height and weight, and which 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 as well. For similar 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 received or are currently receiving radiation therapy
• have low blood cell counts
• have liver or kidney diseases

Schedule (cycles)

Chemotherapy is generally given at regular intervals called cycles. A chemotherapy cycle may involve one dose followed by several days or weeks without treatment. This allows normal cells in the body time to recover from the drug's side effects. Alternatively, doses may be given several 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 several days.

Different drugs work best on different schedules. If more than one drug is used, the treatment plan will specify how often and exactly when each drug should be given. The number of cycles you receive may be determined before treatment starts (based on the type and stage of cancer) or may be flexible, in order to take into account how the treatment affects the cancer and your 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 is important, when possible, to get the full course of chemotherapy and to keep the cycles on schedule. This will give you the best chance to get the maximum benefit from treatment.

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

Go back to Chemotherapy Principles

Last Medical Review: 06/17/2009
Last Revised: 06/17/2009