Perhaps high concentrations of vitamin C are doing something so harmful that it is negating the good that vitamin C is also doing. For example, high dose vitamin C helps prevent a cold or fight a cold, but at the same time, high dose vitamin C inhibits the production of our own antioxidant enzymes, which provide 24/7 protection, a quality of protection that can only be approximated by Pauling type dosing (one dose every waking hour) of vitamin C, which clinical studies never do, though they cannot resist criticizing Pauling without following his protocols.
Another possible harmful effect of high doses of vitamin C: even though free metal ions are low in concentration in the body, high vitamin C drives the formation of a low level of metal ion-vitamin C complexes, which act as non-specific oxidants and do a lot of harm, negating some of the positives done by vitamin C.
Or more specifically, when Cu(II) is bound to DNA rather than a protective peptide sequence, excess vitamin C drives the reduction of any free (non protein or oligopeptide-bound) or DNA-bound Cu(II) to Cu(I), which catalyzes this harmful reaction (compare Fenton chemistry with iron ions) with free hydrogen peroxide:
Cu(I) + H2O2 = Cu(II) + OH- + OH*, where OH* is the reactive hydroxyl radical, which reacts with DNA, particularly with dG, forming 8-oxo-dG.
Note how many levels of breakdown have to occur for excess vitamin C to produce a harmful reaction: why is there excess copper? If excess copper is coming from diet, why did not metallothionein bind it? Why is that excess copper not bound and stored in liver? Why is Cu(II) in cells (other than liver) not bound to protein? Why is Cu(II) bound to DNA? Why is H2O2 being produced near DNA? Why is H2O2 not being turned over enzymatically? Since repaired 8-oxo-dG results in no harm, we can ask, Why was 8-oxo-dG not repaired? Even when vitamin C is doing harm, it is not acting alone: a lot of other things must be wrong.
Or perhaps it is not the amelioration or the prevention of the first cold or insult that is most important, but the second insult. The cold or first insult reduces already inadequate levels of critical cold fighters like vitamin C, leaving a person more vulnerable to any second insult, and there are many possible secondary insults, making these secondary effects hard to study or even to estimate. Even though vitamin C is just one of many cold fighters, a vitamin C supplementer would be better able to withstand the second insult than the non-supplementer. Researchers never test this, not with vitamin C alone or with a group of cold fighting nutrients and they never dose their vitamin C at every waking hour.
I suspect that many diseases may begin locally at a time of weakened local defenses, including at any locale, given outright body-wide deficiency in vitamin C, and the common cold may be all it takes to start something much more devastating.
And by the way, why do IV antibiotics not contain also contain high doses of adjuvant compounds such as vitamin C? A person fighting an infection has markedly reduced vitamin C pools and could suffer a second insult if they are not restored to high levels of vitamin C promptly. And obviously, someone already battling a second enemy like cancer can ill afford the decreased vitamin C that an infection would leave him with.