Drugs are rarely tested on kids. So how do doctors pick the dose?
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One pill for a grown-up, half for a kid? That's not how doses work. Another saying is only half right: that medicines are never tested on kids. Kids do get tested, just less often, and later.
Thirty years ago, over eight in ten American drugs had no section for children on the label. Many Chinese labels say the same: use in children not established. Why so few? Kids can't consent for themselves, and drug makers see little money in it.
So where does a doctor's dose for a child come from? First, what's wrong with half a pill. Picture the body as a bathtub. Taking a dose is pouring water in.
Too little and the medicine does nothing; too much and it does harm. Doctors want the water in the middle band. How big the tub is depends on body weight. That's why doctors ask how much a child weighs, and dose per kilogram. But there's a drain at the bottom, and water keeps flowing out. The drain is the liver and kidneys: the liver breaks the drug down, the kidneys flush it out.
In a newborn, the drain is still tiny. The liver's tools for breaking drugs down aren't finished yet, and the kidneys filter slowly. Give a newborn the adult amount per kilo, and the water keeps rising. That really happened, decades ago: an antibiotic, scaled down by weight, was given to newborns. They couldn't clear it, their skin turned gray, and many didn't survive.
After age one or two, it flips the other way. Now the liver grows fast and is big for the body, so the drain is wider than an adult's. For some drugs, a small child needs more per kilo than a grown-up. So a child is not a shrunken adult: the tub changes, and so does the drain.
So how do doctors know how big the drain is? From small studies done in children. These studies don't put healthy kids at risk. They enroll children who need the drug anyway. Whether it works was already shown in adults. Kids just give a few drops of blood, to see how fast the level rises and falls.
Then computers pool years of dosing records to work out the amount for each age. That water level has a real name, blood drug concentration: how much medicine is in the blood. These numbers go into children's formularies, which doctors check when they prescribe.
The rules are catching up: new drugs in the US and Europe need a plan for studying children, and in May 2026 China called for child doses to be added to labels. So before prescribing, a doctor often asks not how old, but how heavy. Follow your doctor on the dose, and don't split adult pills. On your kid's medicine box, is the dose listed by age, or by weight?
One pill for an adult does not mean half for a child
A child has a fever at midnight. A grandparent snaps an adult fever pill in two and says, "Half is fine for a kid."
That is not how doses work. Before prescribing, a doctor often asks how much the child weighs, not how old they are.
Kids do get tested, just less and later
"Drugs are never tested on children" is only half true. They are, just less often and later.
Thirty years ago, more than 80% of drugs in the US had no children's section on the label. In China, many leaflets still say "use in children not established."
Why so few? Children cannot sign consent for themselves, and drug companies see little money in it.
Think of the body as a bathtub
Taking medicine is pouring water into the tub. Too little and it does nothing; too much and it does harm. The doctor wants the water to sit in the middle band.
How big the tub is depends on body weight. So the doctor asks the child's weight first and works out the dose per kilogram.
Where the picture breaks: the body is not one pool. A drug spreads to different places in different amounts. The "water level" idea still holds.
The tub has a drain
There is a drain at the bottom, and water keeps flowing out. The drain is the liver and kidneys: the liver breaks the drug down, the kidneys flush it out.
In a newborn, the drain is still tiny. The liver's tools for breaking drugs down are not fully built, and the kidneys filter slowly. Give a baby the adult dose scaled by weight, and the water keeps rising.
This really happened decades ago: an antibiotic was given to newborns at a weight-scaled adult dose. The babies could not clear it, their skin turned grey, and many could not be saved. Since then, doctors have been extra careful with newborns.
After age one or two, it flips
Now the liver grows fast and is large for the body, so the drain is wider than an adult's. For some drugs, a young child needs more per kilogram than an adult.
A child is not a shrunken adult: the tub changes, and so does the drain.
The drain size is measured
Whether the drug works was already tested in adults. In children, studies mainly measure the water level, which doctors call "blood drug concentration": how much drug is in the blood.
For some drugs, doctors check the level with a blood test (how blood tests spot problems: see this episode).
The numbers go into a book
These numbers go into a children's formulary, the reference doctors check when prescribing. The rules are catching up too:
No child section on the label does not mean a child cannot take it. Doctors decide from the formulary and studies. This is called "off-label use" and is common in pediatrics.
Don't just split an adult pill
Drugs can also interact with other drugs and food (why grapefruit and some medicines don't mix: see this episode).
Quick check
Before prescribing for a child, what does a doctor most often ask first?
Body weight sets the size of the tub, and most child doses are worked out per kilogram.
