The findings are preliminary and still must be confirmed in humans. And even if the treatment works, it's not a cure but would likely be used in combination with other drugs, the researchers said.

Still, the research does show an unexpected use for vitamin C, which has previously been thought of as a nutrient, not a drug, said study co-author Dr. Mark Levine, chief of the U.S. National Institutes of Health's Molecular and Clinical Nutrition Section.

"There's potential promise that [vitamin C] is part of the armamentarium for treating some cancers," he said. "Which ones? We've got to do more and find out."

Vitamin C has long been one of the most respected of all vitamins, lauded for its supposed powers to treat many ills, from colds to heart disease. The late scientist Dr. Linus Pauling increased the vitamin's profile by touting it as a cancer treatment.

But getting heavy doses of vitamin C into the body is a challenge. Unlike some other vitamins, it's virtually impossible for people to overdose on vitamin C since the body only ingests a certain amount through the mouth and then stops allowing it to build up, Levine said. "The body wants to get to a certain place and no more," he said.

Researchers have found that they can disrupt the body's "tight control" over vitamin C levels by giving the nutrient intravenously and bypassing the digestive system, Levine said. The intravenous approach involves "short-circuiting the body's normal control mechanisms and finding there's an unexpected surprise that may be beneficial," he said.

In the new study, published in the Aug. 4-8 issue of the Proceedings of the National Academy of Sciences, Levine and his colleagues found that intravenous vitamin C produced hydrogen peroxide, which proceeded to reduce cancerous tumors in the mice by 43 percent to 51 percent. The mice had ovarian, pancreatic and brain cancer.

It's not clear why some tumors are immune to the treatment and others are not, Levine said, although normal cells are unharmed by the therapy.

According to the researchers, it's possible to intravenously boost levels of vitamin C in humans to the levels used in the mice.

But Levine cautioned that the treatment isn't ready for prime time with humans. "Should patients with any kind of tumor go out and get IV ascorbate [vitamin C]? That's not the message here," he said.

Instead, he said, the study shows the need for more research.

Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society, said the research is interesting but not yet proven.

"Like so many things that are intriguing or appear to be promising, there appears to be a long way to go from the theory in the lab to the practical application in the clinic."

More information

To learn more about vitamin C, visit the U.S. National Institutes of Health.