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 known as the "generally recognized as safe," or GRAS, standard. That’s not the standard for ingredients introduced to supplements. The standard is not anywhere near that level of scrutiny."

There are some laws regulating dietary supplements, however. In 1994, Congress established the Dietary Supplement Health and Education Act (DSHEA) to address the labeling and safety of supplements, and several more recent regulations mandate that manufacturers observe what are known as "good manufacturing practices," or GMPs, including ingredient testing.

Under the DSHEA, ingredients listed under the "proprietary blend" category don't have to include information about individual amounts of ingredients in that category. Instead, they only have to list the total amount and list the ingredients within the "blend" in order by their weight.

This can be a huge problem when it comes to dosages, as certain amounts of the listed ingredients could have dangerous side effects or negative interactions with prescription drugs.

"From a regulatory perspective they’re all presumed to be safe but the reality is many people….are harmed by supplements," says Cohen.