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Optimistic__Elephant t1_j6fcjaq wrote

Reply to comment by TheRedLob in Can you (roughly) determine the dosage of a drug taken based off of the blood concentration? by bynarie

Somewhat relatedly, how come prescription medicine isn’t sized based on the patient weight?

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TheRedLob t1_j6fqqwt wrote

Some drugs are!

Drugs that work with a single dose for everyone are more convenient. Yes, paracetamol 1g tablet may give lower concentrations in someone weighing 140kg vs 70kg, but these drugs are safe and effective over a wide range of concentrations.

Some drugs have a more narrow therapeutic range. We either adapt the dose by bodyweight (paracetamol for children is a prime example!), but can adapt to many many more covariates. Even race can play a role, with some mutations increasing drug metabolism being less prevalent in whites, so more risk of overdose. And sometimes, it is not the dose but the frequency that is adapted (1g 2 times vs 3 times per day)

In some cases, no covariates exist. You either measure concentration in the blood, or you measure the effect (eg drugs for sedating during surgery).

Sure, it would be more optimal to also dose paracetamol that way. Dose high,measure concentration in blood and adapt. You would get faster pain relief with lower risk. But would you come to hospital and pay a team of doctors/engineers to do that, for a simple headache?

That was the question I answered with my PhD: is all that hastle worth it? For some life saving drugs with strong side effects and very variable metabolism, it is.

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dromaeovet t1_j6gl154 wrote

As a vet, the question of “why aren’t human medications doses based on weight” has always baffled me! We dose nearly everything in mg/kg because patient sizes are so different. We have some rules of thumb for species with a fairly standard size (mg/cat is a common one!) but I typically double check the mg/kg dose anyway.

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sat1vum t1_j6hhp3y wrote

In humans small differences in weight are usually not that big of a deal. Other factors like liver function or wether you take an oral drug with or without food introduce much more variability. Even if weight varies greatly between subjects, it’s often because of a higher fat mass. Most drugs don’t distribute into fatty tissue that well and so in those cases a weight based dose might over dose someone. Fat free mass is sometimes used but often not really needed. In children we do use a weight based dose though.

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bruce_kwillis t1_j6g027h wrote

To add in, at least in the US it's a little less to do with the therapeutic range and a whole lot more with the safety range,and what was tested during clinical testing.

Since we often look at log differences in the clinic when it comes to safety, you can safely assume for most drugs prescribed at x mg/day that they will still be efficacious in a fairly large range of adult weight ranges.

However, especially with some drugs as you have indicated, this isn't the case. Personally I am often worried with something like hormonal birth control, which we are constantly trying to reduce the dose to limit side effects, however there is efficacy issues with larger patients.

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