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Thanks fpr replies. Redcrx, yes it was an allergic reaction to the penicillin. I try not to give him the antihistamine unless the rash comes out, as it makes him very drowsy. Like your little one, he can get the same symptoms - cold-like symptoms/temp & his rash gets really bad if he has a bath, even if the water is quite cool.
His reaction does go after the antihistamine, but I assumed it would not return. Can't keep him on the antihistamine all the time!