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Should Doctors Consider Body Size When Prescribing Drugs?

Q. Shouldn’t doctors take into account body size when prescribing drugs? As a small female, I worry about getting the same dosage of antibiotics as men who weigh 100 pounds more than I do.

A. A one-size-fits-all dosing strategy is reasonable for many drugs. While doctors consider a patient’s weight in some cases, weight is generally subsumed into larger considerations of therapeutic index and titration when determining the right dose for a patient.

The therapeutic index is the ratio of a drug’s efficacy to its toxicity. Put simply, it is the margin of safety, or the amount of a drug that causes the desired therapeutic effect compared to the amount that causes toxic side effects. This margin is determined early in the process of drug testing during Phase I trials, which are designed to test the safety of a wide range of doses. Because these trials are a mandatory step in the Food and Drug Administration approval process, each drug’s therapeutic index is well defined.

Many antibiotics have a sufficiently high therapeutic index to allow for a single dose to be used. Such an approach is reasonable because the goal of treatment is eradication of an infection rather than determining the lowest effective dose. Treatment of severe infections and patients with serious medical problems may require personalized dosing. But for the routine infections that constitute the bulk of office-based practice — such as urinary tract infections, sinusitis and bronchitis — one dose is generally reasonable regardless of body weight. It is also reasonable for many over-the-counter allergy, pain and acid reflux medications.

A patient’s weight and other attributes — such as kidney function, liver function and variations in metabolism — come into play for drugs with a lower therapeutic index. Here doctors use the principle of titration: Start with the lowest effective dose, then slowly increase the dose until the desired effect is achieved. Titration is the norm for asthma medications, which are adjusted according to symptoms. Other titrated drugs include drugs for diabetes, which are titrated to blood sugar levels; thyroid medications, which are titrated to thyroid function; and the anticoagulant warfarin, which is titrated to a lab test for blood clotting.

Children are the special case in which weight-based dosing is always required. This is because weight can vary a hundredfold from preemies to teenagers versus perhaps fivefold among adults. And there is little margin for error.

Thus, in their daily prescribing practices, doctors do adjust for weight, although they don’t always make this adjustment explicit to their patients. Fortunately, the F.D.A.’s guidance ensures that physicians stay within ranges that are generally considered safe.

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