Thousands of men across the United States, Puerto Rico and Canada taking part in a study to see whether vitamin E or selenium is decreasing the occurrence of prostate cancer are now being urged by the National Cancer Institute to stop taking the supplements.

The organization, which helped organize the study, further added that the men would continue to have their health monitored for about three more years.

The study, known as “SELECT” or Selenium and Vitamin E Cancer Prevention Trial, involved more than 35,000 men age 50 or older who were given either the supplements or dummy pills. More exactly, the participants were assigned to one of four groups: taking selenium along with vitamin E; selenium with a vitamin E placebo; Vitamin E with a selenium placebo; and selenium and vitamin E placebos. The study began in 2001 and was scheduled to run until 2012.

Previous research has suggested that the two supplements might prevent prostate cancer. Both of them are antioxidants, compounds that interfere with chemical reactions that can damage cells and DNA. In 1998, a study in Finland showed that more than 29,000 male smokers who took vitamin E to prevent lung cancer had 32 percent fewer occurrences of prostate cancer.

The new study comes to contradict previous findings, as it showed “two concerning, but not statistically significant trends: there were slightly more cases of prostate cancer in men taking only vitamin E and slightly more cases of diabetes in men taking only selenium,” the National Cancer Institute, one of the National Institutes of Health, said in a statement.

However, “neither of these findings proves an increased risk from the supplements,” meaning it could be a coincidence, the statement further read.

Prostate cancer is the ninth most common cancer worldwide, but is the number one non-skin cancer among American men. It seems that this type of cancer affected 18 percent of US men and led to death of 3 percent in 2005 only. According to the estimates of the American Cancer Society, about 186,000 men will be diagnosed with prostate cancer by the end of this year. Almost 28,000 men loose the battle with the disease each year. Over 90 percent of prostate cancers are spotted when it is still confined to the prostate, a gland in the male reproductive system, or has extended to close parts of the body within the pelvis.

The American Cancer Society says men should get tested annually for prostate cancer with Prostate-Specific Antigen (PSA) screening test, and digital rectal examination (DRE) once they reach the age of 50. But testing is recommended to men at high risk starting the age of 45. Men at high risk are considered the African-Americans and those whose relatives have had prostate cancer. About two-thirds of all prostate cancers are diagnosed in men age 65 or more.