system
Given the following post, respond as if you're commenting on Reddit:
human
Why are all adults prescribed the same dosage antibiotic regardless of weight?

I recently ran a course of cefdinir, which was the first time I had taken this particular antibiotic. As I was poking around online reading about it I noticed that dosages for children under 12 are based on weight. So many mg per Kg. That leads me to my question as to why adults aren't also dosed like this. It seems a 150lb man would be prescribed an identical dose as a 500lb man. So why aren't dosages based on weight, and are equal dosages equally impacting across a wide spectrum of adult body types?
gpt
Children have immature organ systems and have to be dosed accordingly. In theory a child's body weight should be fairly proportional to their actual growth development. Adults regardless of size should have a fairly standard efficiency in regards to organs such as liver and kidney clearances. A 500 lb man is going to mostly be that heavy based on adipose (fat) tissue which doesn't contribute to clearance or tolerance levels to medications. So in theory that 500 lb and 150 lb adult should be able to tolerate the same amount of medicine up to a maximum safe and effective  amount. Now on the flip side we do see adjustments in certain medications based on age and previous medical conditions that might compromise these same organ systems in an otherwise healthy adult. Geriatric dosing is normally less due to decreased rate of clearance.

TLDR : Children have immature organ systems whereas all healthy adults should have close to the same clearance rates despite weight as their organs are fully developed.