LONDON - Gonorrhea, a common sexually transmitted infection, can double the risk of bladder cancer in men, researchers said Tuesday.

Earlier studies already had suggested a link, and scientists from the Harvard School of Public Health in Massachusetts who monitored the health of 51,529 American men found 286 cases of bladder cancer in men who had had the infection.

"We observed a two-fold increase in bladder cancer risk among men with a history of gonorrhea," said Dr. Dominique Michaud, the lead author of the research reported in the British Journal of Cancer.

The link was stronger for invasive and advanced bladder cancer, which is more serious and difficult to treat, and among smokers.

Bladder cancer is the ninth most common cancer worldwide. Smoking is a leading cause of the disease and accounts for 65 percent of cases in men and 30 percent in women, according to the International Agency for Research on Cancer in Lyon, France.

"Gonorrhea is an infection that often recurs, causing local inflammation and symptoms such as incomplete emptying of the bladder," Michaud said. "The inflammation itself or the associated symptoms could be contributing to the development of bladder cancer."

Professor John Toy of the charity Cancer Research UK said the findings strengthen the suspected linked between gonorrhea and bladder cancer in men.

"The next step is to confirm whether the increased risk could be caused directly by the gonorrhea infection or its symptoms," he said in a statement.