In a British trial examining the benefit of selenium supplements it has been found that in some cases the supplement can do more harm than good.
The research originally set out to test whether selenium supplements could prevent skin cancer and was conducted by Dr. Saverio Stranges of Warwick Medical School.
Some earlier research has suggested that the mineral might help prevent diabetes.
Dr. Stranges and colleagues looked at 1,202 people taking selenium for the skin cancer trial who did not have diabetes at the beginning of the study.
Half of the study group took a daily 200 microgram selenium supplement and half received a placebo pill for an average of 7.7 years.
The researchers found that 58 of 600 people taking selenium and 39 of 602 taking placebos developed type-2 diabetes over the 7.7 years; this equates to an increase in relative risk of about 50 percent.
The researchers say people taking selenium in the hope of preventing diabetes may actually increase their risk.
The trial showed that people who took selenium pills raised their risk of diabetes by more than half, compared to similar people taking placebos.
It is thought about 60 percent of Americans take multivitamin pills, many of which contain between 33 and 200 micrograms of selenium, in addition to the selenium taken in from food and the air.
Selenium is a naturally occurring trace mineral present in soil and foods; minute amounts are needed by the body to help the metabolism.
Selenium supplements are widely promoted on the Internet for a wide range of conditions from cold sores and shingles to arthritis and multiple sclerosis.
They are also promoted as preventing aging, enhancing fertility, preventing cancer and getting rid of toxic minerals such as mercury, lead and cadmium.
Dr. Stranges says he would not advise patients to take selenium supplements greater than those in multiple vitamins and says most people have adequate selenium in their diet.
Dr. Stranges says it's not clear as yet why selenium increases the risk of developing diabetes.
The research is published in the Annals of Internal Medicine.