Researchers gave 41 patients with pre-malignant mouth lesions green tea extract orally for three months at three doses. Others took a placebo.
The patients were followed for more than two years. Fifteen developed oral cancer at the end of that time period.
The study found that about 59 percent of people taking the highest dose of the green tea extract showed a clinical response, compared with 18 percent of those who took a placebo.
In end, there was no difference in oral cancer development overall between those who took green tea and those who did not. But patients who had abnormal cell growth at the start of the study took a longer time to develop oral cancer when they took green tea extract.
Still, researchers caution against any recommendations that green tea can definitely prevent cancer.
"The goal of this kind of research is to determine whether or not these supplements have long-term prevention effects. More research including studies in which individuals at high risk are exposed to these supplements for longer time periods is still needed to answer that sort of question," said study author Vassiliki Papadimitrakopoulo in a written statement.