Extensive marijuana or cocaine smoking might increase the risk of lung cancer, suggests a small study that compared tissue samples from people who smoke tobacco and illegal drugs.

Scientists at the University of California, Los Angeles, found that smoking marijuana and crack can cause the same precancerous changes in their bronchial cells that tobacco smoking causes well before the smoker gets cancer.The study, reported in this week's Journal of the National Cancer Institute, also found that smoking both tobacco and marijuana or cocaine increased the risk, because those smokers were more likely to sustain additional precancerous changes. Also, these combination smokers were more likely to have damage to their p53 gene, an important gene in fending off cancer.