"The effect of the antioxidant was dose-dependent, which means that if we gave a low dose, tumors increased a little bit, and if we gave a high dose, tumors increased a lot," Bergo said.

The researchers also looked into human lung cancer cell lines and found that the antioxidants increased the growth of the cells, which were cultured on plastic plates.

"So those findings suggest at least the possibility that these findings will be applicable to humans with lung cancer," Bergo said.

The antioxidants seemed to boost cancer progression by decreasing the amount of a key tumor suppressor protein called p53, though they can also reduce levels of reactive oxygen species that can harm cells. When p53 in the mice and in human lung cancer cell lines is knocked out, the antioxidants will have no effect, the researchers said.

The findings suggested that people carrying small undiagnosed tumors in the lungs, which is possible in anyone, but more likely in smokers, should avoid taking extra antioxidants because they may accelerate tumor progression, though more studies are needed.

"So the take-home message of this study is that antioxidants may be harmful and should probably be used with caution by patients with lung cancer and people with an increased risk of developing the disease, such as smokers," said co-author Per Lindahl, professor of the University of Gothenburg.

The researchers said that they will next test other types of antioxidants and will also look into whether these findings apply to other types of cancer.