A new study suggests that patients with breast cancer who take additional vitamins during chemotherapy(化疗) treatment may face increased risks.

    Researchers said the use of dietary supplements(补充剂) that increase levels of antioxidants, iron, vitamin B12 and omega-3 fatty acids appeared to lower the effectiveness of chemotherapy. Researchers reported their findings in theJournal of Clinical Oncology.

    Christine Ambrosone is the head of cancer prevention and control at the Roswell Park Comprehensive Cancer Center in Buffalo, New York. She said, "From this study and others in the literature, it seems that it may not be wise to take supplements during chemotherapy."

    "It's thought that antioxidants might interfere with the ability of chemotherapy to kill cancer cells," Ambrosone said.

    Some doctors have been advising patients for a number of years not to take antioxidants during chemotherapy. "But there was no strong empirical data for that recommendation," Ambrosone said.

    So, Ambrosone and other researchers decided to study whether supplement use might affect chemotherapy's effectiveness. They looked for evidence in an earlier study on diet, exercise, lifestyle and cancer results.

    In the earlier research, people who took part were asked about their use of supplements at the beginning of and during treatment, and about their lifestyle, diet and exercise. The researchers studied 1,134 patients who filled out the surveys and followed them for a median of six years.

    The researchers searched for possibilities that might increase the risk of the disease reappearing or of death. They found that patients who took any supplements at the beginning of and during chemotherapy were 41 percent more likely to have their breast cancer return than those who did not. In addition, the supplement takers were 40 percent more likely to die later on compared to patients using no supplements. The supplements included vitamin A, C and E. Those taking vitamin B12 and iron supplements were at greater risk of cancer returning, the researchers said.