Max appropriate dose of cetirizine for sudden onset intensely itchy rash?

[cardamomginger]

DH has woken up with a intensely itchy hot and bumpy rash over much of his torso. We have just got off the phone with the dr at the OOH who was pretty unhelpful. She thinks it is an allergic reaction and has told DH to take antihistamines. We have cetirizine. She would not say what an appropriate single dose is for such reactions - she said to 'look at the box'.

Not helpful. And doctors out there? How much can he take?

She said if it gets worse to go to A&E. Otherwise GP. Which is fine. But he needs something to help now!

Doesn't it say on the box?

5mg, can be increased to 10mg.

The box just says for general use. It doesn't say what the maximum safe dose is for sudden-onset pretty much total torso bright red itchy allergic rash. The box says 10mg per day. I've been prescribed 20mg twice per day. So more than a single tablet is safe and in some cases is indicated. I'm pretty certain that DH is one of those!