The researchers found that mice with tumours given the pills grew three times as many and as large tumours as the mice in a control group, who were not given the supplements, Swedish Radio News reports.
Martin Bergö, professor in molecular medicine at Sahlgrenska Academy in Gothenberg, and one of the researchers behind the finds told Swedish Radio News: "If you already have a tumour, antioxidants might be dangerous."
The study looked at mice who had already developed lung cancer.
"Our study shows what happens with extra antioxidants", Professor Martin Bergö added, "both mice and humans get antioxidants through their food, but what we have done is to give them extra antioxidants. The human equivalent would be eating food supplements with high levels of antioxidants in them."
He adds that this research is important: "There are doctors around the world that recommend alternative medicines, to take antioxidants to protect against cancer, even after a cancer diagnosis. That really needs to be looked at", he told Swedish Radio News.