Nicotine can prevent chemotherapy drugs such as Taxol from killing lung cancer cells, researchers reported on Sunday in a finding that may help explain why lung cancer is so difficult to treat in smokers.

They said their findings may also suggest that even people who quit smoking but use nicotine supplements, such as patches or gum, may not be helped as much as they should be by cancer therapy.

“Our findings are in agreement with clinical studies showing that patients who continue to smoke have worse survival profiles than those who quit before treatment,” the researchers wrote in a study published by the Proceedings of the National Academy of Sciences.

Srikumar Chellappan and colleagues at the H. Lee Moffitt Cancer Center and Research Institute in Tampa, Florida, tested three standard lung cancer drugs: gemcitabine, cisplatin and Taxol, on several different batches, called cell lines, of cells taken from lung cancer tumors.

Adding a small amount of nicotine, equivalent to what would be found in the blood of an average smoker, interfered with the drugs’ action against the tumor cells, they reported in the study, which was also presented to a meeting of the American Association for Cancer Research.

Nicotine protected the cancer cells by increasing the activity of two genes called XIAP and survivin, Chellappan’s team found. The genes stopped a process called apoptosis, a kind of cell suicide.

When the two genes were suppressed, the cells died as they should have, they said.