Dosage for 3y9m of piriton for chickenpox.

[Shortfatandangry]

Can anybody advise dosage for piriton for my son, who'll be 4 in March. He has chickenpox and there's no pharmacy open to ask advice.

It's says one 2.5ml spoonful every 4-6 hours on the box.

Probably should have read that first eh?!... sorry, assumed it was the 6y+ as that's what he's been prescribed in the past for allergies. Thanks for your help.