Every medicine has two numbers, and neither of them is the dose.
The first is the amount below which nothing happens: the drug is in the body, but too thinly spread to do the work it was given for. The second is the amount above which it stops being a treatment and starts being a poisoning. The gap between those two numbers is what doctors call the therapeutic window, and prescribing is the art of landing inside it.
For most medicines the window is comfortably wide. A little more or a little less makes no practical difference, which is why a painkiller can be sold with a single instruction on the box.
For a few, it is narrow — narrow enough that patients taking them have their blood tested regularly, because the difference between the useful amount and the dangerous one is smaller than the difference between one person's body and another's. The same dose is right for one patient and wrong for the next.
What the window really describes is a set of permitted values. Outside it the quantity is not merely a poor choice; it is the wrong kind of number altogether — one that the situation cannot accept. Every formula that describes something real carries a window like this, and the values outside it are not answers at all.