More than 40 percent of American adults are considered obese, or medically overweight. But the medications many of these individuals take are rarely tested in bigger bodies.

Obese people are currently not required to be included in drug studies. Often, they are purposefully left out.

This is a problem because the amount of drugs an obese person should take to be safe is unknown, says Christina Chow. She is a researcher who has sought to bring attention to the need to include obese people in drug development and testing. "There's no real emphasis for them to be studied at all," Chow told The Associated Press.

Medications can work differently in people who are obese, but exactly how and at what amount is not clear. Research suggests this includes antibiotics and antifungal drugs, medications used to treat serious infections. It even includes ibuprofen, a commonly used painkiller that can be bought without a doctor's orders.

The U.S. Food and Drug Administration (FDA) and the National Institutes of Health (NIH) are the agencies that control and provide money for drug testing. At a gathering last year, FDA Commissioner Dr. Robert Califf noted a "deficit of evidence" about how medicines act in patients who are obese. The NIH now asks researchers to think about the results of not including obese people in their studies, an agency spokesperson said.

At a recent medical conference, Chow presented a review of more than 200 studies for new drugs in the U.S. last year. Nearly two-thirds of the studies failed to include weight or body mass index, a numerical value noting if a person is obese.

Historically, some populations have not been included in drug testing to avoid harm, including pregnant women and children. In the past, women, racial and ethnic minorities and aged people have also not been included.

Dr. Caroline Apovian is a researcher at Brigham and Women's Hospital in Boston. She is the co-author of Chow's study. Apovian told the AP there are many reasons people who are obese are not included in drug tests.

She noted that people who volunteer for trials are often thin and healthy and this does not effectively represent the general population. Obese individuals can have many health difficulties, and researchers often worry this will affect results of their work.

"Sometimes patients with obesity have many more comorbidities than others. They'll have more diabetes, more heart disease, more strokes," Apovian said.

But if drugs are not studied in a condition that affects 42 percent of the U.S. population, the real-world effects can be dire, experts say.

Some drugs remain in the body longer in obese people. That could result in harmful drug interactions if another medication is added too soon. There can also be less medication in the blood, leading to undertreatment, Apovian said.

Rexulti is a drug often used to treat people with schizophrenia or depression, Chow noted. Research has shown that in obese patients it may take much longer to reach the amount of Rexulti needed to be effective. As a result, many patients and their doctors may stop treatment too early or believe the drug does not work.

"Not treating or undertreating schizophrenia may be dangerous to themselves and the people around them," Chow said.

Even a drug as common as ibuprofen may not reduce pain in people at higher weights when taken as directed, research shows.

But without testing and clear directions, doctors won't know the amount of medicine needed for obese people, said Dr. Colleen Tenan. She is a board member of the Association of Clinical Research Professionals. Tenan said it is very hard to be a doctor and direct patients to take amounts of medication that have not been tested.

Change is coming, but progress is slow, said Dr. Alison Edelman. She is an obstetrician and researcher at Oregon Health & Science University. She told the AP that in 2019, the FDA suggested that researchers permit obese women in studies on birth control. Even though the suggestions are not required, they have already changed how she and other researchers structure their studies.

"Because unless we see representation in our study population, we don't end up with treatments that work well for individuals," Edelman said.

Apovian, from Brigham and Women's Hospital, said patients can ask their doctors about whether the usual amount of a medication is acceptable for their weight. Even if the doctors do not know, the question could start an important discussion about the most effective treatments.

"This is a big issue," Apovian said. "It can be important for patients to speak up."

I'm Gena Bennett. And I'm Bryan Lynn.

Jonel Aleccia reported this story for The Associated Press. Gena Bennett adapted it for VOA Learning English.

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emphasis –n. special importance

comorbidities - n. two or more diseases or medical conditions in a patient

dire – adj. extremely serious or urgent

schizophrenia – n. a disease that affects the way the brain processes information and causes a loss of touch with reality