When taken orally, massive quantities of vitamin C are either broken down or excreted unused by the body. In contrast, intravenous administration of vitamin C produces blood levels 100 to 500 times higher than oral administration.

This could explain why many clinical trials on vitamin C and cancer — most of which have used oral administration — failed to support the results seen in laboratory studies using cancer cells.

Surprisingly, the most recent study on the topic from researchers at the University of Iowa (UI), published in the journal Redox Biology, suggests that vitamin C's cancer-fighting potential might come not from its antioxidant capabilities, as previously assumed. On the contrary, vitamin C appears to generate free radicals that tear apart cancer cells while leaving healthy cells unharmed. 

Targeting a weakness of cancer cells

In prior research, the same scientists found that high-dose vitamin C selectively killed cancer cells in the laboratory, and had the same effect in mice … if the vitamin C was administered intravenously. In the new study, researchers sought to discover the causes behind this effect.