Are Prescription Drug Doses Based on Patient Weight?

All drug dosages must account for the weight of the patient, because weight plays a role in the distribution of the drug in the body's tissues. Prescription drugs are no exception to this rule, but many are designed to be suitable for patients in an “average” weight range, so that precise dosages do not need to be calculated. This is designed to make it easier for patients to use the drugs, and to make it easier for pharmacists to package them. In some cases, weight may be an important factor in the prescription, in which case the doctor will discuss it with the patient.

In the hospital, medical staffers can calculate drug doses very precisely. There are many drugs which are primarily used in the hospital, and they include dosage recommendations which usually indicate the weight of the patient in kilograms, as in “60 milligrams per kilogram.” When these drugs are prescribed, the doctor must do the math to calculate the dosage correctly, and the administering nurse or doctor may check the calculation to confirm that it is correct.

In the case of prescription drugs, asking pharmacists to formulate precise drug doses is not always practical, and sometimes patients have difficulties adhering to a drug regimen if they are obligated to deal with calculations about drug dosage. Liquid medications can be more easily adjusted for patient weight, which is why many drugs for children come in liquid form, so that a doctor can prescribe “two teaspoons daily” or “10 cubic centimeters (CCs) twice a day.” This flexibility is key when one considers that the same drug might be used to treat infections in toddlers, young children, and adolescents: obviously, an adolescent dosage would be way too high for a toddler, while a toddler dosage would be useless for an adolescent.

Pills may also be periodically adjusted to accommodate patient weights which fall outside the normal spectrum. When patients are asked to take half a pill or a pill and a half a day, it reflects a desire on the part of the doctor to achieve drug doses which are as close as possible to the recommended amount. With some drugs, a pharmacist can also create a custom compound which does allow for very precise dosage calculations, but compounding pharmacists are increasingly rare.

There are two concerns with drug doses. The first is that in an unusually slender patient, a normal dosage might be too much, increasing the side effects for the patient and potentially contributing to patient discomfort. Slender patients may also be at risk for an overdose in some cases. In larger patients, the issue is that the drug may become too diluted, necessitating a dosage which is somewhat larger than that calculated to suit patients in the normal range.

Communicating with a doctor about how a drug is working is critical, as it is possible to adjust drug doses if a drug is not working for a patient. It is also important to follow directions from a prescribing doctor very closely, to ensure that the dosage is correct. Patients who are unclear about how to administer a drug should not be afraid to ask for a demonstration from their doctors.