A new study helps explain why taking antioxidants may accelerate the growth of early tumors or precancerous lesions in high-risk populations, like smokers. Well-known antioxidants include vitamins A, C and E, as well as some medications. They are chemical compounds that delay some types of cell damage by preventing the buildup of molecules called reactive oxygen species, or ROS. For a long time scientists thought antioxidants could be useful for preventing cancer, but recent clinical trials in humans have hinted that antioxidants do not prevent lung cancer, and may actually increase cancer risk in certain high-risk groups. The reason for this effect has been unclear, however. Studying two different antioxidants, vitamin E and a drug called acetylcysteine, antioxidants sped up the progression of lung cancer in mice and in human cell lines.