Why are medications often dosed based on weight for children but not for adults?

Question

I was looking at the information that came with my medication, and I noticed that the starting dose for children and adolescents under 70 kg is 0.5 mg/kg, or for children and adolescents over 70 kg, and for adults regardless of weight, it's 40 mg. But what about adults that are significantly under 70 kg? It seems to me that my response to medication would be a lot more similar to that of my 16-year-old sister, who's close to the same weight as I am, than, say, my dad who's well over twice my age and probably close to twice my weight. My doctor had me take only 18 mg (which still produced enough side effects that I stopped taking it after four days), but I'm curious as to why the recommended dosages for many medications seem to assume the response in adults won't be weight-dependent when it clearly will be.

Answer 1

First, the weight of children varies to a much greater extent than the weight of adults. For example, an average 10-year old weighs ten times as much as an average newborn.

Second, there are many different medications where getting exactly correct blood concentrations is very important, because too little could mean no effect, too much could cause toxic effects. These medications have to be dosed based on weight, even in adults. Examples include heparin (a blood thinner), gentamicin (an antibiotic), cyclosporin (an immune suppressive medicine) etc. However, for most frequently used medications, the therapeutic range is quite wide, meaning that getting a little higher or lower blood concentration (that could happen if your weight is a little more or less) does not make a difference in the effectiveness or in the side effects.

Two more things to take into consideration:
1. People who are overweight may be obese (have a larger amount of fat but the same amount of muscle tissue) or can be more muscular than "small" people. Certain medications are stored and get concentrated in tha adipose (=fat) tissue, while other medicines are only distributed in the water content of the body. For medicines that do not get into the fat tissue, having more body fat does not make a difference.
2. The blood level of many medications depends much more on their rate of elimination (kidney function, liver function) than on the volume of distribution (which correlates with body size).

Answer 2

Super question!

The amount of medications you get is based on your weight so that you do not get an overdose. An overdose in children or adults can lead to brain damage, mental retardation, or even death.

It is a bit odd that doctors and pharmacists assume your weight. I believe that the doctors have chunks of ages or age groups (say 150-160 lb.). This could be a new technique, or who knows, it could even be a glitch.

Glad to help!
-Science Tigger

Answer 3

Very good question. The average medication is for a person weighing 150 pounds. Doctors and pharmacists should tell their patients to adjust medication according to their weight.
I imagine that it would be difficult to print all the dosages for the product, as the type on most drug packages is already very small. So they only print the average person size dosage.

Answer 4

they assume an average weight for an adult. For children they are still growing and the best way to gauge it is by their current weight. Once you know how hard meds will affect you as an adult you can adjust it yourself (through docs advise) to a lower or higher perscription. but most is based on an average that adults arnt growing anymore and are around a certain weight.

Answer 5

both bottle water and water filters are legal alternatives. you have the right to refuse treatment, but you might have to get off your but to do it. laws are for the greatest good, we don't banish peanuts because a few are allergic. Fluoridation is primarily for the purpose of preventing tooth decay in children and children do not have a right to refuse treatment, only adults. as a responsible adult, figure out a way to quit crabbing about it, or move to Colorado where the water has fluoride naturally and it is not an artificial additive.

Answer 6

Basically, children contain more water than an adult. This increases Vd (volume of distribution) and therefore makes the drug more dilute. Also, children differ in their rates of metabolism and excretion for drugs. You can look in any pharmacology textbook to get a better understanding of Vd.

Answer 7

Generally speaking adults reactions to drugs (inclding absorbing/excreting etc) are not weight dependant.

These effects are dependant on how well developed our bodies are.