[ELI5] Why are medications dispensed at very specific/seemingly almost spontaneous solution/dosage amounts?

Question

I am sure there is a reason why. But, I have always wondered if a slight variation would change things.

For example. A bag of sodium chloride 0.9% is commonly dispensed at hospitals and clinics. Why is it *specifically* 0.9%? Would there be a completely different effect if it were, say, 0.10%? Or 0.85%? Or did people just realize this amount happened to work and it became default?

I also have seen medications of random components that will be like *insert chemical* 0.15% or a medication like colchicine at 0.6 MG. Why isn’t it 0.5 MG??

Just curious. Hopefully not a stupid question.

In: Chemistry

Answer 1

First, just a minor pedantic thing: 0.10% is not larger than 0.9%, but much smaller, so you’d definitely see a huge difference there. I think you meant to say 1%.

About your actual question: In your specific example, the 0.9% are the result of trying to match (or actually, slightly exceed) the ideal salt content of human blood. It’s basically the same effect as when a drink says it’s “isotonic” – it means that it has just the right concentration of minerals for your body. This value is something we can just measure, there is a clear “right” answer of what the human body contains by default and then some practical considerations why it might make sense to slightly exceed that number (e.g., you usually give sodium chloride to people who’s sodium levels are already low, so it makes sense to slightly overshoot).

More generally though, most medical dosages and concentrations were established in some kind of medical study/trial where different dosages were tested and compared, and they went with whichever worked best. Usually there is either a trade-off between achieving the intended effect of a drug and avoiding unintended side effects. It is not always the case that the recommended doses are technically optimal (especially not if you consider that not all humans are the same), but it’s “good enough” and only will be officially replaced if newer studies show that a different dosage is even more “fine-tuned” and shows better effects overall.