Guaifenesin should be listed under the active ingredients, it is an expectorant that may cause a user to vomit at high doses.
Also, frequent use of dextromethorphan (as with many other substances) can most certainly lead to a psychological addiction. Frequent heavy use may also put the user in a state of delerium or "dxm psychosis". A friend of mine began doing DXM (Robo and Coricidin) nearly 5 times a week or more. After he quit his job and dropped out of college he started reporting that the "dxm gods" were inside of him and later that he was being haunted by himself from the past. He eventually had a revelation; that "the past doesn't matter anymore" and at some point started cutting back.