I hear this from parents all the time — and they are right!
A study in the online version of Pediatrics found that pharmacies often dispense insufficient liquid antibiotic to complete an antibiotic course. If thirty 1 tsp doses are to be given over ten days, the doctor usually orders 30 tsps of medicine. The pharmacist correctly mixes up 30 tsp of medicine. But medicine sticking to the side of the bottle or spoon (and measuring devices of varying accuracy) means that up to 30% more medicine would be needed to complete the course in the real world.
If people are diligent in giving the doses, they run out of medicine after an average of only 8 days!
The article suggests dispensing both measuring devices and extra medicine — the amount of medicine depending on the thickness of the liquid. I like the idea of dispensing measuring devices to parents (most pharmacies do not do this), but I rather think that the extra medicine is not important for most people.
Much of our experience with these medicines is based on the amounts real people have been giving — not on the theoretic amounts. The ‘8 day’ ten-day-course may even be the better plan.