Although researchers have tested the vitamin as a cancer therapy many times, they almost always concluded that it was ineffective—but they were guaranteeing failure by giving it orally to patients.

When it's given intravenously, it bypasses the gut and goes directly into the bloodstream—where concentrations of the vitamin are up to 500 times higher than when it's taken orally—and targets cancer cells, say researchers at the University of Iowa.

The therapy is now going through the approval process, and could soon be available as an alternative to chemotherapy or radiation, the two conventional cancer treatments.

It's been proved to be effective in animal studies, and phase 1 trials have demonstrated that it's safe and well-tolerated.

Now doctors at the university are starting to use it on patients with pancreatic cancer and lung cancer, and are measuring their progress against other patients who will continue to be given chemotherapy or radiation.

Biologist Garry Buettner, who works at the university, has worked out just why vitamin C is so effective: the vitamin breaks down quickly in the body, and generates hydrogen peroxide that kills cancer cells. "Cancer cells are much less efficient in removing hydrogen peroxide than normal cells, so cancer cells are much more prone to damage and death from a high amount of hydrogen peroxide", he explained. "This explains how very, very high levels of vitamin C do not affect normal tissue, but can be damaging to tumour tissue."