I've been on 6mp for some years now and have not really experienced any different levels of susceptibility to colds, or worse colds when I do get them. While the immunomodulators do turn our immune systems down a bit, in my experience the effect is negligible as related to catching or revovering from other bugs. Since a cold usually runs a week or so, I suspect you can do the Vit-C without doing any harm to the immunosuppressant effect. My GI has always told me to just handle other illnesses as i always have without worrying about
the 6mp factor.