People with cancer who are taking antioxidant supplements may be compromising the effectiveness of their chemotherapy, an expert has warned.

Professor Kathryn Steadman, a pharmacist and cancer researcher from the University of Queensland, said the primary mechanism of action of many chemotherapy agents was the generation of reactive oxygen species.

“Antioxidant supplements may inhibit reactive oxygen species, thereby protecting cancer cells from death,” she warned in a short report published in this week’s MJA.

A systematic review published in 2007 showed no evidence that antioxidants decreased the effectiveness of chemotherapy but many of the studies were small and underpowered, she said.

The patients included in the review also had advanced or relapsed disease rendering the findings “not applicable” to patients with earlier disease, Professor Steadman added.

She conceded that the jury was still out on whether antioxidants had a positive or negative effect on adjuvant cancer treatment.

However given the narrow therapeutic window of cancer drugs, “relatively small changes in dose intensity that may occur due to antioxidant activity could compromise treatment effectiveness,” she said.

“This is especially important in the curative-intent patient population, because reducing the effective chemo- therapy dose intensity may adversely affect disease-free and overall survival,” she concluded.