Name: Why Can't Kids Just Take Smaller Doses of Adult Meds?
Uploaded: 2020-03-30T03:21:54.000Z
Duration: 6 min 48 s
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If you've ever strolled through a pharmacy in search of over-the-counter meds,

you might have noticed that lots of drugs have special children's formulas.

And you might think that's because smaller people need smaller doses.

But you'd be wrong. Because kids aren't just tiny adults.

In fact, children sometimes need larger doses of medicine!

Welcome to the weird science of allometry.

Allometry simply refers to the study of how physiology changes with size,

and it plays a big part in understanding how much medicine kids should get.

You might think bigger bodies would need larger doses of medicine

since they have more of whatever tissue or cells the drug is targeting.

that are prescribed based on a patient's weight

what's sometimes referred to as a weight-based dose.

For instance, an adult might be prescribed five milligrams of medicine

But what's really weird is that this dose isn't constant

You often can't take the dose per weight an adult would take

and simply multiply it by the child's weight to figure out how much they should get.

And that's largely because a person's cells use less energy as their body grows.

In other words, bigger people have lower mass-specific metabolic rates

Even at the cellular and subcellular level,

oxygen and calorie consumption is just slower in larger bodies.

Not only that, but it's a nonlinear relationship

so as bodies get smaller, metabolism increases fast.

And that matters when it comes to medicines

because it impacts how fast your body processes drugs.

Generally speaking, the faster the metabolism, the faster you break down a drug

and that means, even if you're small, you might need to take more

to have the amount you need stick around long enough for it to work.

Let's consider acetaminophen, which you might know as Tylenol.

It's an over the counter drug you can take to reduce pain and fevers.

The regular adult dose for acetaminophen is 650 milligrams.

So an adult weighing roughly 80 kilograms

the average mass for adults in North America

would generally take that amount every four to six hours as needed.

If you'd never heard of allometry, you might think a child one-quarter that weight,

20 kilograms or so, would get 162.5 milligrams.

The child will metabolize it super quickly,

so the drug won't really have the chance to be effective.

With junior strength tablets, the recommended dose is actually 240 milligrams.

And for a 44 kilogram child, the recommended amount is 640 milligrams.

That's basically the same as what's recommended for adults,

even though the child is about half the weight!

And if you calculate the weight-based doses for everyone,

the children actually take about two times as much.

which is an antiviral used to treat influenza infections,

So, a little under two milligrams per kilogram per day for our 80 kilogram adult.

But pediatric doses range from three to six milligrams per kilogram per day!

As for why metabolic rate scales with mass in this strange, nonlinear way, well…

But by looking at people and animals of all sizes,

they have come up with some good hypotheses!

The first and perhaps most obvious is that

little bodies belong to individuals that are still growing,

and it takes more energy to build tissues than to maintain them.

because the allometric pattern holds across different sized adults, too.

That's where the shapes of smaller and larger bodies might be coming into play.

Because a smaller body has a larger surface area relative to its tiny volume,

and, therefore, run a higher metabolic rate, to stay warm.

But even that still doesn't fully explain the allometric scaling

because it only makes sense in so-called “warm blooded” species like us

that strictly regulate their internal temperature with the heat they produce.

And this metabolic rate scaling occurs in reptiles

and other so-called “cold-blooded” species, as well.

There may be another way that overall shape impacts all this, though,

Some mathematical work suggests metabolic rates in large animals

See, branching blood vessels have to strike a balance between their width,

the pressure of the blood moving against their walls,

and the force it takes to move that blood around.

In a bigger animal with a larger, longer, and branchier circulatory system,

Since it's slower and the body is larger,

it takes longer for blood and the oxygen it carries to get places.

And the animal can't beat its heart faster or harder to speed things up,

because if it tried, its heart would fail.

its cells simply can't keep using oxygen at the same rate.

More work is needed to sort out how all these ideas apply to humans specifically.

But whatever the reason for children's elevated metabolisms,

the effect on drug dosing can be pretty dramatic.

So, ultimately, it's not that there are separate formulations

If anything, it's so that a patient, or their caregiver