^Jackie - they were saying they no longer using CCC and were just mentioning it to explain that even though it contains an antihistamine they still had the same red blotches. (EDIT: wrote that before the OP replied)
. . . . .
Red blotches on the face or other areas seem to be a pretty common side effect of DXM.
There seem to be 2 types.
One is a temporary itchy red rash resulting from the histamine release which normally goes away pretty quickly and doesn't seem to occur in users of Corcidin Cough and Cold (NOT that using CCC is a good idea, just mentioning it as it is evidence that it's due to a histamine release).
The other type sounds more like what you have, longer-lasting, more noticeable red blotches. I'm not sure exactly what causes it, this could still be due to a very strong histamine release, and allergies/histamine
is the most common cause of red blotches in general (including among non-DXM users, some people get it from makeup, sun, inhaled allergens, etc). It could be some sort of a true allergic reaction to the DXM or one of its inactive ingredients, but you said when you took CCC you still got it. I'm thinking it could possibly be due to broken capillaries or something like that, especially since you said it lasts for weeks afterwards.
Does anyone know if DXM causes blood vessels to dilate? I know some people get red skin or red blotches from alcohol, which dilates the blood vessels.
You could try adding an antihistamine (again) to see if it helps, although I'm not sure whether it would actually be a wise idea to take diphenhydramine with such high doses of DXM. Maybe a very small dose of diphenhydramine could be ok, but I'd be very careful because of interactions like possible serotonin syndrome, increase in adverse side effects, etc. Does anyone on here have personal experience taking diphenhydramine with such high doses of DXM? Another option is an antihistamine spray, which you would apply to the skin and so would be safer.
Of course, what I would do is not take such high doses of DXM
EDIT: Another "cause" of red blotches on the face is a condition called Rosacea, which is more of a set of symptoms as opposed to a specific cause. There are a few theories as to what might cause it, but the underlying cause(s) of Rosacea have not been proven. Rosacea may be a vascular disorder because of its association with flushing, redness and visible blood vessels. Some doctors have speculated that flushing may involve the nervous system, since rosacea is often triggered or aggravated when patients are under stress or take certain drugs. One theory about swelling is that increased blood flow during flushing leads to an increase in tissue fluid, which accumulates faster than the lymphatic system can remove it. The swelling in turn may contribute to skin thickening as tissue accumulates. So if what's happening to you is
not due to a histamine reaction and more of a drug-induced Rosacea, this could be something to consider.
(sorry this was so long!)