NEW YORK -- One in three adults in the U.S. is taking both prescription medications and dietary supplements, creating a risk for dangerous interactions, according to a new study.
Multivitamins with added ingredients like herbs or fish oil were the most common form of supplement mixed with medications, researchers found.
"Multivitamins are commonly assumed to be safe, but our analysis suggests multivitamins, which may include multivitamin 'plus' combination products, can also contain botanical and herbal ingredients that have the potential to interact with prescription medications," Harris Lieberman told Reuters Health in an email.
Lieberman, the study's senior author, is a researcher with the Military Nutrition Division of the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts.
Lieberman said the team did this study to determine how many people in the U.S. are using dietary supplements and prescription medications together, and whether patterns of dietary supplement use are different among people with various kinds of medical conditions.
"This information can help health care professionals to identify who may be at risk of having an adverse interaction between a supplement and prescription medication," he said.
The researchers focused on 10,480 adults (4,934 women who weren't pregnant and 5,016 men) who answered survey questions about their dietary supplement and prescription medication use, as well as whether they had any of the following medical conditions: asthma, arthritis, congestive heart failure, coronary heart disease, angina, heart attack, stroke, high blood pressure, high cholesterol emphysema, chronic bronchitis, cancer, weak bones or problems with the liver, thyroid or kidneys.
The researchers found that 47 percent of participants diagnosed with any of those medical conditions used both supplements and prescription medications. That compared to about 17 percent of adults who didn't have those conditions, but were taking prescription medications for other reasons, such as birth control pills or antidepressants.
Overall, 34 percent of the participants -- representing some 72 million people in the U.S. -- were taking some kind of dietary supplement along with a prescription medication, according to the results published in the Journal of the Academy of Nutrition and Dietetics.
Cardiovascular medications were most likely to be used along with dietary supplements, followed by central nervous system agents, hormones, metabolism-related drugs, psychotherapeutic agents and antibiotics or antivirals.
Multivitamins containing other ingredients were more common than standard multivitamins. The ingredients most often added to the enhanced multivitamins included fish oil, botanicals, herbs, probiotics, fiber, enzymes, antacids and glucosamine and chondroitin.
Supplement use was most common among people with diagnosed osteoporosis, followed by those with thyroid, cancer, arthritis, cardiovascular, kidney, diabetes, respiratory and liver conditions.
The authors call the findings "concerning" because some herbal supplements are known to alter the way the liver metabolizes drugs, and can increase or weaken the potency of a medication.
Annette Dickinson told Reuters Health that the large number of people who used both supplements and prescription medications in the study didn't surprise her.
Dickinson is a consultant for the Council for Responsible Nutrition, a supplement industry trade group, and an adjunct professor in food science and nutrition at the University of Minnesota.
She wasn't involved in the new study, but has researched some of the reasons why consumers take dietary supplements.
"Obviously anybody who is taking prescription medication should be telling their doctor everything they're taking so that a judgment can be made whether there is, or might be, an issue," she said.
Dickinson added that pharmacists may also be a good source of information on medications and dietary supplements, but she doesn't think that should be a substitute for speaking with a doctor.
"Patients, especially those taking medications or given new prescriptions, should always inform their doctors about what dietary supplements they are taking, and doctors can help patients by asking about their supplements," Lieberman said.
Lieberman added that if a patient is concerned, then bringing the supplement's original container will help doctors and other healthcare providers identify ingredients in supplements that have a potential to interact with medications.
"Patients can also use reputable sources to check if there have been reports about interactions of dietary supplements they are considering taking and their medications."
The National Institutes of Health MedlinePlus website, for example, has information on interactions between drugs, supplements and herbal ingredients.