From:
Erowid DXM Vault - DXM FAQ - 6.1.3 Mild allergic reactions
6.1.3 Mild Allergic Reactions and Histamine Release
Category: Frequent
Ah, the "Robo Itch". Some people get it and some don't. There's evidence that at least some of the cases of Robo Itch are a psychological reaction to mild anesthesia, but many are probably a result of histamine release - not necessarily an allergic reaction per se, but a possible consequence of DXM's pharmacology. The itching tends to go away, and although scratching is pleasurable (and a loofah is wonderful), take care not to overdo it.
Actual allergic reactions have occurred, and often these are a result of the "inert" ingredients, usually one of the dyes (e.g., tartrazine). A topical antihistamine spray might be a good idea. You should always keep an oral antihistamine on-hand, at least during your first few DXM experiences (or when trying out new preparations).
Just remember not to use any prescription, non-drowsy antihistamine with DXM. Diphenhydramine (BenadrylTM) is a good OTC antihistamine that is probably safe to combine with DXM (at least it hasn't caused anyone problems yet).
People who have used CoricidinTM tablets have reported that the Robo Itch seems to be completely absent. Be warned, however, that one should not use DXM+antihistamine tablets above the second plateau due to the potential for adverse anticholinergic effects (which if nothing else tend to cause bad trips).
Note that some people find the itching to be unpleasant enough to keep them from trying DXM again, and a few people have scratched themselves raw. Do take care.
