Not sure if DH's logic is right?

[CraneBoysMysteries]

Mishap with the shopping in the week. DH was asked to get calpol (or own brand substitute). Both DCs are fighting colds and we've just had double chicken pox so are out of it. Like to keep some in stock

DH went to Morrisons and got 3 boxes of own brand.

But...

...accidentally got 6+. He can't find the receipt and has already opened one bottle. Both kids are under 6.

His logic is that the dose of paracetamol per 5ml is just more than double. So we can use the bottles but give just less than half the usual dose and be mindful of 24 hr max on the original calpol.

This...sort of makes sense to me?? But I'm not sure if it's that simple?

Both bottles screenshot and the only active ingredient is paracetamol,

YANBU- logic is sound. If usually giving 5ml, move to 2ml of the over 6s

YABU- it's not that straightforward and here's why....

For medicine calculations it is "what you want" divided by "what you've got" multiplied by "what it is in"

So if you want the equivalent of the 5ml dose of the baby calpol then you want 120mg. Divide 120 by 250 (which is what you've got, the 6 plus) and then multiply by 5 (because the 6 plus is 250mg in 5ml so 5 is "what it is in").

This gives 2.4ml. So 2.4ml of 6 plus is the same as 5ml of infant.

Your husband is right for paracetamol/calpol. The most accurate way as previously mentioned is by weight. However, as also mentioned it is not safe to do this for all medicines as children's bodies process different drugs in a non linear manner and an adult dose cannot be just halved for a child, or as my 12yo ds who is bigger than me still can't have lemsip as his ginormous body will not process the decongestant aspect in the same way as mine does.

What's that got to do with anything?
If the 6+ dose is twice as 'strong' just half the dose. Simple arithmetic

no idea how it works with paracetamol, but with many drugs, the amount does NOT raise in proportion to weight, at this stage, depends on state of development of liver, nothing to do with weight. Not in any way possible to calculate it with arithmetic

Yes, there is. If, for example, 6+ calpol is double the strength of infant calpol, then half a dose of 6+ calpol is the same strength as a full dose of infant calpol.

True that for some medications dose will be related to factors order than weight but the fact remains that if you know the dose for the patient then you can calculate that dose from a variety of concentrations. The OP is not proposing that she gives her under 6 a full 6 year old dose, she is calculating yes nine of mls of 6 plus she would need to give her under 6 in order to give their usual dose and this is absolutely fine, it is the same dose of medication just in less fluid.

You're correct @Nimbostratus100 until your last sentence. Someone else has already done the leg work so it can be calculated. They've considered a weight range and calculated safe doses for that range. The MHRA have approved them. Clearly it won't be the exact perfect dose for both the children as one is presumably bigger. Likewise if a child is at the top end of the range they don't suddenly need 2.2 times as much. However the doses stated are safe soo it IS simply arithmetic. Pharmacist do this all the time. Run out of 400mg tablets. Take 2 200mg. I've had a swig of Calpol myself and gauged 4 teaspoons (or 1 adult tablet)