My question relates to the recommended dosages of over-the-counter drugs (non prescription), mainly painkillers like ibuprofen, aspirin and paracetamol. Intuitively I have always thought that the recommended dosages and dosage intervals would have to essentially guarantee minimal undesirable side effects on the smallest and most frail of the categories listed for dosages, child or adult.
I always imagined that the drug would be more widely dispersed and diluted as the total amount of body’s blood/fat/bones/tissues increased with changes in body size and composition.
Perhaps out of ignorance, it occurs to me that if a 50kg, skin-and-bone old man can handle 400mg ibuprofen every 4 hours to satisfactorily relieve a headache, then a 120kg 6’2" chubby dude requires much more ibuprofen for the same effect. Even if the drug worked through a system in their bodies which was in different proportions to just the ratio of their masses, the brain for example, the big dude still usually has slightly more of everything (receptors for drugs to bind to).
Do I have my understanding wrong, or could the dosages of these drugs be recommended by body weight or total body fat amount or other?
Each drug has its own particular profile. Some drugs distribute through the body by weight (and are prescribed by mg/kg measures. Others take no notice of body weight as their presence in the body is heavily controlled by internal processes- the liver etc.
Drugs are not single action, and one action that is desired may have a different threshold to another that is not (side effects). Additionally, individuals react differently to drugs because of their own metabolism.
It would be a foolish thing to do to use folk science theories to decide maximum dosages.
This is especially the case with acetaminophen (Paracetamol) which has a low therapeutic ratio- the potentially lethal dose is frightenenly close to the normal Therapeutic dose.
Thank you, Pjen, I’ll now modify what I was going to write, in accordance with what you pointed out:
Some meds say “Take 1-2 capsules every 4-6 hours.” My guess is that those are the ones where body size and/or severity of the condition factors into the dose. Others say “One capsule every four hours,” and those are the ones which are not so flexible.
IANAD, but as someone once pointed out on this message board, for acetaminophen (and probably for other drugs as well), it’s not about the size of your body, it’s about the size of your liver. Just because you’re a big guy, that doesn’t mean that your liver can process the drug more quickly or efficiently. Please consult with a doctor before taking more acetaminophen than the prescribed dosage. I’ve often felt that the one situation in which the Dope has indisputably saved lives is when people have come in saying, “I’m a really big guy, so I take [some large amount] of Tylenol, because I need that much to be effective,” and are told that that is a really bad idea.