Although dietary supplements are often touted for their health benefits, they may in fact increase your cancer risk, especially if taken in high doses, according to a new analysis of previous research.

In the analysis, researchers looked at 20 years of published studies on supplements and people's risk of cancer.

"In a nutshell, the answer is no, the vitamin pills do not reduce cancer risk," said the author of the analysis, Dr. Tim Byers, of the University of Colorado Cancer Center.

For example, in a study published in 2006 in the American Journal of Clinical Nutrition, researchers found that women who had a high intake of folic-acid supplements had a 19 percent greater risk of breast cancer than those who did not take such supplements. Moreover, women who had the highest levels of folate, the water-soluble form of folic acid, in their blood had a 32 percent greater risk of breast cancer than those who had the lowest levels.

"The reason the whole line of research began in the first place was that we observed that people who eat more fruits and vegetables seem to be at a lower cancer risk," Byers said. Researchers wondered whether there were particular vitamins in fruits and vegetables that were driving this beneficial effect, he said. [10 Do's and Don'ts to Reduce Your Risk of Cancer]

"And the really scary finding is that in several of the trials, we see an increase in cancer in groups" that were taking supplements such as beta carotene, vitamin E and folic acid, he said.

In a study of more than 35,000 men, published in 2011 in the journal JAMA, researchers found that taking high-dose vitamin E supplements was linked to a 17 percent increase in cancer risk over seven to 12 years.

In another study, published in 1994 in the New England Journal of Medicine, male smokers who took beta-carotene supplements had an 18 percent higher risk of lung cancer over five to eight years compared with male smokers who did not take the supplements. The finding "raises the possibility" that beta carotene supplements are harmful to smokers, the researchers said in their article.

"I think these are substantial enough increases in risk that we really need to consider them as both a clinical and public health safety issue," Byers said.

Many people in the United States take high-dose vitamin supplements thinking it will improve their health, "when it fact it could be just the opposite," he said.

The results of the new analysis were presented today (April 20) at the American Association for Cancer Research (AACR) Annual Meeting 2015.

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