For certain medications with a narrow therapeutic effect (that is, the beneficial dose is just below the toxic dose), the dosage is calculated on a body-weight basis. Examples include some drugs used to treat cancer, many pediatric drugs and certain antibiotics, such as gentamicin and vancomycin (Yancocin), In general, many factors other than weight are used to determine the dose of a drug, including kidney and liver function.. .the stage of a disease being treated...and other drugs the patient may be taking. For some medications, the dose is based on the actual level of the drug in the blood. But for many medications, the therapeutic dose is far below the toxic dose, and a fixed dose can be used for most patients.

Caution: In some cases, the lowest dose commercially available is still too high for older patients, due to reduced kidney and/or liver function and other physiological changes. In these cases, the drug is administered less often, or the dose is cut in half if the medication is suitable for splitting.

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