Scientists are afraid of the possibility that a Vitamin C function could prevent the action of certain types of chemotherapeutic drugs. Vitamin C destroys free radicals. Some cancer drugs work by generating free radicals designed to kill cancer cells. This article contains a test tube study that illustrates this effect. ref
Now, test tube studies isolate cancer cells and just add one or two nutrients to measure influence. But a method has to be found that measures if this Vitamin C action also occurs inside the human body. The ethics involved in this type of study make it difficult to use humans as test subjects for this theory. Using test tube studies may not exactly reflect what happens in the body due to the lack of synergism of the many other nutrients also present and the large number of other processes happening at the same time. There have been cases where the exact opposite really occurs once inside the body. OSU Pauling Institute> ref
SIDEBAR: The effectiveness of vitamin C to neutralize free radicals is really what is being measured here. And yes, vitamin C provides this function so Scientists are concerned this vitamin C action would limit chemotherapeutic drug generated free radicals. Thus, it might be prudent to limit vitamin C to food sources during chemo episodes, But not necessarily during other periods. --end
When Scientists observed the action in question after the addition of the cancer cells to mice, greater credibility was generated for this possibility. ref Now, remember the first article in this series talks about how vitamin C prevents tumor growth at higher amounts. This is not in anyway jeopardized by this chemo drug interference action. Here is a good review of vitamin C actions from the American Cancer Society. ref
One aspect that needs clarification concerns the fact that higher blood levels of vitamin C from food sources are protective against cancers while supplements of vitamin C do not seem to provide this protection.
Perhaps the anti-oxidant loads from supplements of vitamin C have a greater chance of crossing a balance point which foods cannot achieve plus foods also have many other valuable phyto-nutrients, maybe some of which have synergistic actions.
Do higher doses of vitamin C interfere with the action of cancer cell killing chemo-drugs? They might! But maybe not for all cancers or even all chemotherapy drugs. And this action could depend upon the level of antioxidants from other sources.
NOTE: Since vitamin C has many important functions, the body does need some. Science should determine the optimal level with the most benefits and least possible adverse effects, using a synergistic method and under different situations of cancer types and treatment protocols.