Study author Dr. Richard F. Branda cautioned that chemotherapy patients should first discuss taking supplements with their doctors, because some supplements may interfere with treatment. For instance, cod liver oil and St. John's Wort may interfere with blood thinning drugs, hormone treatment or chemotherapy.

Previous research has also shown that an herbal dietary supplement that some men use to treat prostate cancer, called PC-SPES, may interfere with the anti-cancer activity of the chemotherapy drug paclitaxel, making it less effective.

However, studies have also shown that vitamin E may enhance the benefits and reduce the side effects of chemotherapy, and many doctors now recommend vitamin therapy during treatment.

To investigate whether supplements help reduce side effects from chemotherapy, Branda and his colleagues asked 49 women with breast cancer to complete questionnaires detailing their use of supplements during chemotherapy.

The authors found that more than 70 percent of the women were taking at least one of 165 different types of supplements. On average, patients took three supplements. However, some women said they took up to 20 daily supplements during treatment.

The most common supplements were multivitamins, vitamin E and calcium.

Women who took multivitamins or vitamin E alone experienced a smaller decrease in their neutrophils during chemotherapy.

However, women with relatively high levels of the B-vitamin folate in their blood had a larger-than-average decrease in neutrophils, the authors report in the journal Cancer.

Branda, who is based at the University of Vermont in Burlington, explained that many cancer patients - and people without cancer - take supplements because they believe they are "natural," and could therefore only help them.

However, Branda noted that supplements typically consist of complex chemicals, which can have many possible effects on the metabolism of drugs and the functioning of cells. "These effects may be beneficial or detrimental and need to be studied further," he said.

For instance, based on the results with folate, Branda recommends that cancer patients avoid taking extra folate if they eat a balanced diet, because many foods are already fortified with folate, or folic acid.