Many people gobble big doses of vitamin C in hopes of boosting their immune system and warding off illness. But new research shows that in people with cancer, the vitamin may do more harm than good.

Researchers at Memorial Sloan-Kettering Cancer Center in New York studied the effects of vitamin C on cancer cells. As it turns out, the vitamin seems to protect not just healthy cells, but cancer cells, too. The findings were published today in the journal Cancer Research.

“The use of vitamin C supplements could have the potential to reduce the ability of patients to respond to therapy,” said Dr. Mark Heaney, an associate attending physician at the cancer center, in a press release.

Dr. Heaney and his colleagues tested five different chemotherapy drugs on cancer cells in the laboratory. Some of the cells were first treated with vitamin C. In every case, including a test of the powerful new cancer drug Gleevec, chemotherapy did not work as well if cells had been exposed to vitamin C. The chemotherapy agents killed 30 to 70 percent fewer cancer cells when the cells were treated with the vitamin.

A second set of experiments implanted cancer cells in mice. They found that the tumors grew more rapidly in mice that were given cancer cells pretreated with vitamin C.

The researchers found that just like healthy cells, cancer cells also benefit from vitamin C. The vitamin appeared to repair a cancer cell’s damaged mitochondria, the energy center of cells. When the mitochondria is injured, it sends signals that force the cell to die, but vitamin C interrupts that process.

“Vitamin C appears to protect the mitochondria from extensive damage, thus saving the cell,” Dr. Heaney said. “And whether directly or not, all anticancer drugs work to disrupt the mitochondria to push cell death.”

Dr. Heaney measured the buildup of vitamin C levels in cells and said that the levels of vitamin C used in the experiments were similar to those that would result if a patient took large doses of the vitamin in supplement form. Earlier research at the cancer center showed that vitamin C seems to accumulate within cancer cells more than in normal cells.

Patients should eat a healthy diet that includes foods rich in vitamin C, Dr. Heaney said, but it’s the large doses of vitamin C in tablet form that are worrisome.