The subject pretty much sums up my question, but I’ll elaborate a little. I very occasionally will take a Claritin to deal with Pet allergies and it strikes me as strange that it recommends a dose of 1 pill per 24 hours for all adults. Now, I’m a adult male who’s 6’3" and 220 pounds. It would be very common for people ranging +/- 100 pounds of my weight to be taking the same dose. Why is this?
Some medications have smaller doses for children under 12, or some other age bracket, but they never seem to mention body size or gender. I understand that not all medications are contingent on body mass for dose, but I have to imagine that some would be. I have never personally seen any medication, OTC or prescription, that assigns a recommended dosage that varies by the size of person. Presumably a doctor could prescribe a different pill size or concentration based on your size if it were relevant to that specific medicine, but never have I seen a conditional dosing regimen.
What gives? I don’t believe that alcohol is the only substance out there that metabolizes differently for people of varying sizes. I’m sure the explanation is probably some ridiculous legal CYA, but I figured I’d see if anyone here knew differently.
Body mass would be one easily measured variable that will effect drug metabolism. But there are numerous other ones - the metabolic enzymes that clear drugs from your system can vary quite widely between populations and individuals. As it would be impractical to construct a genetic profile for everyone who wants to buy an OTC medicine, the approach taken is to make the drug as safe as possible.
The pharma company will be aiming for a big a therapeutic window as possible (see here for a related definition), greater than 100. If the drug doesn’t have a good safety window then it won’t generally be on sale OTC. Plenty of drugs that have narrow safety windows are administered in hospitals, where the exact plasma concentration can be administered. The blood thinner warfarin is a good example of this. OTOH you still get things like paracetamol (acetaminophen in the US) sold OTC which can be hazardous, its therapeutic index must be way lower than 100.
Does that answer your question? I guess the short form is that body mass is just one parameter effecting drug metabolism and can never be considered on its own as the single guiding principle for dosing.
Offhand, I recall seeing some OTC medications that did specify different doses for different body weight ranges for children–ok, I know you said body mass, but body weight would translate close enough for those purposes. I’ve never seen it done (or don’t recall it if I have) for adult dosing, though.
The fact that there are other factors doesn’t explain why the issue of weight is disregarded by pharmaceutical companies. If weight is a big one, and an easy one to gauge, then it seems they should account for it. If they are playing it safe by assigning a low dose to accommodate 100 pound women then they would be compromising their effectiveness for a large portion of the market. You have to assume that this would hurt overall sales if every guy over 200 pounds had what seemed to be limited effectiveness due to a too low dose.
That index seems irrelevant. Generally speaking, the ratio is going to be the same for all people, because the “effective dose” and the “toxic dose” will both vary based on weight by similar proportions. The fact that X dose of Medication-A will help a 100 pound person and a 3X dose will kill and hundred pound person is irrelevant if X is linearly proportional to that persons weight. If anything, disregarding the weight of the subject would make dosing more unpredictable and cause the concentrations in a person to dance all over that window of acceptability.
No one is saying it should be treated as the only measure. But if it’s a reliable and easily deduced measure it seems foolish to obscure it from both a safety perspective and a sales perspective.
Well, yes, we are on Earth so I think it’s safe to use the terms interchangeably.