"I could swear I've heard that the thing about body mass affecting the effectiveness of medication was a myth, but I'm not sure where I heard it."
It definitely affects it: This is part of the reason they ask you for your weight before you get an MRI with contrast (The amount of contrast changes on your weight) and IIRC, they change the dose of IV morphine depending on your weight. Realistically, this is also the case for some drugs - a small person generally gets drunk faster than a large one.
But it doesn't matter as much for some drugs than others: Two adults can generally take the same amount of ibuprofen because the differences aren't really all that large. Extremes are going to change this, though.
It definitely affects it: This is part of the reason they ask you for your weight before you get an MRI with contrast (The amount of contrast changes on your weight) and IIRC, they change the dose of IV morphine depending on your weight. Realistically, this is also the case for some drugs - a small person generally gets drunk faster than a large one.
But it doesn't matter as much for some drugs than others: Two adults can generally take the same amount of ibuprofen because the differences aren't really all that large. Extremes are going to change this, though.