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Flickr/Mike Mozart

On the back of virtually any bottle of supplements are two words that strike fear into the heart of Pieter Cohen.

The words are printed inside a small square on the back of the bottle beneath bold lettering which claims to list the supplement’s ingredients: “Proprietary blend.”

Under the protective umbrella of these two words, a supplement maker does not have to list all of the details of what’s in its product, according to Cohen, an assistant professor of medicine at Harvard Medical School.

This “allows companies to put in ingredients without telling us the amounts,” says Cohen, who spoke on a recent panel put on by The Forum, an event series organized by the Harvard T. H. Chan School of Public Health. “And those tend to be the higher-risk product.”

The risks can be serious. Several supplements have been linked with an increase in certain cancers; others have been tied to an elevated risk of kidney stones. Although this research has been widely published, supplements continue to send roughly 20,000 people to the emergency room every year.

Cohen likes to compare the safety framework for supplements with that for food. “In food,” says Cohen, “they have to meet what’s…