WEDNESDAY, Oct. 1 (HealthDay News) -- Large doses of vitamin C could reduce the effectiveness of anticancer drugs, according to a new study that focused on laboratory cancer cells and mice.

The finding raises questions about whether human patients might suffer the same effects, the study authors said.

"There's a possibility that taking supplemental vitamin C could have a detrimental effect on cancer treatment," said study author Dr. Mark L. Heaney, associate attending physician at Memorial Sloan-Kettering Cancer Center in New York City.

However, there's no indication that smaller doses of vitamin C, such as those found in food and ordinary multivitamins, might be a problem, he said.

Vitamin C has not traditionally been considered a hindrance to cancer treatment. In fact, some scientists -- including the late Linus Pauling -- have long viewed vitamin C as a potential cancer fighter. Recent research has suggested that vitamin C could help prevent cancer, but it's not clear why.

The vitamin, found in fruits and vegetables, also has a reputation as a treatment or preventive for a variety of ailments, including colds.

In the new study, Heaney and his colleagues gave doses of vitamin C to mice with tumors before they underwent chemotherapy. While the type of vitamin C given to the mice isn't available over the counter, it's the equivalent of a 2,000-milligram dose for humans, Heaney said. That's the equivalent of the vitamin C found in 75 six-ounce glasses of orange juice. Supplements packing that much vitamin C are found in health stores, he said.

The researchers found that the vitamin C reduced the effectiveness of the chemotherapy by about 30 percent to 70 percent.

Heaney theorized that it's possible that vitamin C may disrupt chemotherapy's killing processes and provide protection to the cancer cells.

The findings were published in the October issue of the journal Cancer Research.

Heaney said the next step is to launch studies with people, but, he added, it may be a "hard sell" to convince patients to take high-dose vitamin C supplements if it appears they counteract chemotherapy.

For now, he said, "What I recommend to my patients is that they continue to eat a well-balanced diet that has vitamin C, and that they don't take supplemental vitamin C. I think a multivitamin is fine."

Dr. Chí Van Dang, who directs the Institute for Cell Engineering at Johns Hopkins University School of Medicine, said the new study appears valid, "although some conclusions are based on small numbers of animals. A larger number of animals could settle whether there is a real trend or not."

Scientists should take the study seriously, he added, but the other side of the equation must be examined, too. "Additional studies are necessary regarding whether vitamin C as a single agent could prevent cancer or the recurrence of cancer once treated."

Pamela Mason, scientific adviser and spokeswoman for the Health Supplements Information Service, said, "The study concluded that vitamin C reduced the effectiveness of anticancer drugs in laboratory cell cultures and in mice with implanted cancer cells. Though the researchers said that their findings could have implications for human beings treated with anticancer drugs, they also added that this needs to be tested in a proper clinical trial."

More information

Learn more about vitamin C from the Linus Pauling Institute.