The medicines you take contain a soup of active and inactive ingredients.

Active ingredients are the ones that provide a therapeutic benefit, while inactive ingredients are just that — inactive — meaning they don't react in the body and are instead there to enhance the properties of the medication itself, such as its taste, appearance and ability to be absorbed by the body. [7 Bizarre Drug Side Effects]

But it turns out that inactive ingredients may not be as, well, inactive as we think: A new study finds that, in some patients, inactive ingredients can trigger allergic reactions or other symptoms of food intolerance.

The study was published today (March 13) in the journal Science Translational Medicine. (Of note, three of the study authors hold a patent on a system that examines the burden of inactive ingredients in pills.)

The researchers began looking into inactive ingredients after senior study author Dr. Giovanni Traverso, an assistant professor in MIT's Department of Mechanical Engineering and a gastroenterologist at Brigham and Women's Hospital, treated a patient with celiac disease who had a reaction to a medication that contained inactive ingredients derived from wheat products.

Traverso and his team started by looking through the medical literature for reports of patients reacting to inactive ingredients.The researchers found some studies on patients who had allergic reactions to inactive ingredients such as lactose — present in about 45 percent of pills — as well as certain kinds of chemical dyes.

But they didn't find any studies looking at whether certain inactive ingredients may cause less extreme, but likely more common symptoms of food intolerance, such as bloating or stomach aches.

The researchers also dug into data about inactive ingredients themselves, using a database run by the National Library of Medicine. They found that active ingredients make up, on average, just over a quarter (29 percent) of the weight of an oral pill; the remaining 71 percent of the weight comes from inactive ingredients. On average, a pill contains more than eight different inactive ingredients but can contain up to 35, they reported.

But far than 35 inactive ingredients are available. In fact, drug companies have around 1,000 inactive ingredients to choose from when manufacturing pills, the researchers found. Of these ingredients, 38 of them, such as peanut oil, lactose and some dyes, are known allergens. But though most inactive ingredients are usually tested to see if they're toxic — and though they're found to not have a major effect on most of the population — these toxicology tests may miss certain small side effects in some people, according to the report.

The study found that 93 percent of pills contain at least one of the 38 allergens and that almost all of them contain substances that could be problematic for people with certain food intolerances, such as gluten or sugar.

Still, not all experts are convinced that inactive ingredients are particularly problematic.

Dr. John Kelso, an allergist and immunologist at Scripps Health in San Diego, California who wasn't involved in the study, doesn't see a cause for concern.

"Such reactions are quite rare," he said. "In most cases, the amount of the food protein [present] in the medication would not be sufficient to trigger an allergic reaction."

For example, the amount of egg protein that's present in flu shots is not enough to trigger reactions even in people with severe egg allergies allergic to eggs, Kelso told Live Science. Indeed, the the American Academy of Pediatrics revised recommendations for flu shots containing eggs, saying that it's no longer necessary to ask people if they are allergic to eggs before giving them the flu vaccine, because the risk is so minimal, he said.

As such, "for the vast majority of patients with [a] food allergy, there is no reason for them to avoid medications with [inactive ingredients] derived from the foods to which they are allergic," Kelso said.

He did note, however, one inactive ingredient in that may cause problems for people with allergies: gelatin. That's because some medications and vaccines are either administered intravenously or injected can contain larger amounts of gelatin, and can trigger allergy reactions. Therefore, these drugs should be avoided for people with gelatin allergies, but Kelso added that "even most of these patients tolerate [taking] gelatin in capsule form."

Originally published on Live Science.