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Why don’t doctors adjust for age and size when prescribing drugs? © iStock

Why don’t doctors adjust for age and size when prescribing drugs?

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Don't be fooled when a doctor is looking you up and down, they are only dealing with your dosage.

Asked by: Sam Lowry, Leamington Spa


They do, though exactly how depends on both the patient and the medicine. For example, elderly patients often have liver and kidney function issues, which reduces the dose they can tolerate. In contrast, babies and toddlers are so small that their response to drugs depends critically on their weight, sometimes unpredictably. Even the gender of the patient can play a role in deciding dose.

Then there’s the type of drug: some are fat-soluble, so the correct dosage depends on Body Mass Index (BMI) – that is, total weight divided by the square of height - while other drugs are water-soluble, where dosage is better based on lean body mass. Yet another complication comes from the so-called therapeutic index of a drug – roughly speaking, how big a dose can be given before benefit turns into toxicity. Calculating the right dosage then depends on striking a balance between giving a high enough dose to tackle the condition – but not so high that it does more harm than good.

According to Professor Donald Singer of the British Pharmacological Society and President of the Fellowship of Postgraduate Medicine, this is often the case with cancer therapy and some powerful antibiotics, and getting the dosage right for a specific patient can be very challenging.


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Robert is a science writer and visiting professor of science at Aston University.


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