Americans are taking more prescription medications. They also are taking more supplements — everything from vitamin and mineral pills to fish and flax seed oils. The natural result: More are combining drugs and supplements. That may be riskier than many consumers realize.

Some are risking dangerous internal bleeding by combining certain supplements with blood-thinning drugs. Others are unknowingly reducing the effectiveness of medications they take to fight cancer, control infections or prevent pregnancy.

“It’s a serious concern, and the risk is growing,” says Dima Qato, an assistant professor of pharmacy at the University of Illinois in Chicago. Qato led a recent study, published in JAMA Internal Medicine, documenting increased mixing of all sorts of medications and supplements among U.S. adults ages 62 to 85. The study found that 87% used at least one prescription medication, 38% took at least one non-prescription drug, and 64% used at least one supplement. Many took multiple drugs and supplements.

The researchers looked for combinations known to the be dangerous and found that the most common involving a supplement was a mix of blood-thinning medication warfarin and supplements of omega-3 fish oils. The combination, which increases bleeding risks, was taken by 8 in 1,000 older adults in 2010-2011, up from 1 in 1,000 in 2005-2006. The researchers also found about 4 in 1,000 combining niacin supplements with the cholesterol-lowering drug simvastatin, which can increase side effects.

In a statement, Andrea Wong, vice president of scientific and regulatory affairs at the Council for Responsible Nutrition, which represents supplement makers, said the results “show that the prevalence of older adults taking a combination of drugs and supplements with a potential for interaction is low, suggesting that the actual risk to consumers is low.”

But Qato says that “the trend is not reassuring” and that the analysis included only the most-used supplements and drugs and the most serious known reactions. Many combinations have never been studied, she says: “Their safety is largely unknown.”

Chun-Su Yuan, director of the Tang Center for Herbal Medicine Research at the University of Chicago, agrees: “Patients need to know that there are potential adverse events. They don’t know. And they assume that these products are natural, so they are naturally safe, but that’s not always the case.”

The consequences might be particularly serious for patients with life-threatening illnesses such as cancer. Some supplements are known to make certain chemotherapy drugs less effective or to increase their side effects, says K. Simon Yeung, a research pharmacist and acupuncturist at Memorial Sloan Kettering Cancer Center in New York. Others could have unknown effects, he says: “Cancer patients should know that many of these products have not been studied in a cancer care setting.”

All the experts, including those at the supplement industry group, urge consumers to discuss the supplements they take with their health care providers. A separate report published alongside Qato’s study found that 25% of adults using herbal medicines and other supplements did not do that. Doctors also have a responsibility to ask patients about supplement use, the experts agree.

Health care professionals and patients can find information about most supplements at the website of the National Center for Complementary and Integrative Health, which is part of the National Institutes of Health. Memorial Sloan Kettering also has a website, About Herbs, which details potential uses and risks of herbal remedies and other supplements.

Here’s what you need to know about several popular supplements. (All information is from About Herbs, unless otherwise noted):

Possible benefits: Prevention of bone loss, cancer and heart disease (study results are mixed, especially for cancer and heart disease).
Possible drug interactions:

May reduce absorption of some medicines, including bisphosphonates (for osteoporosis) and certain antibiotics.

Possible benefits: May reduce cholesterol and blood pressure (study results are mixed).
Possible drug interactions: May increase bleeding risks with anti-clotting drugs (blood thinners). Reduces effectiveness of the anti-HIV medication saquinavir. Lowers blood sugar, so may affect insulin needs.
Note: In most cases, herbs used in cooking are not a concern, but some reports have documented increased post-surgical bleeding in patients eating large amounts of garlic.

Possible benefits: Studies suggest it help relieve nausea and vomiting. It may also stimulate appetite and ease digestion, but studies are lacking.
Possible drug interactions: May increase bleeding risks associated with anti-clotting drugs. May affect insulin needs. May reduce blood levels of cyclosporine antibiotics.

Omega-3 fatty acids (including fish oil)
Possible benefits: May protect against cardiovascular disease (though the NCCIH says study results have been mixed). May decrease some cancer and mental health risks.
Possible drug interactions: May increase bleeding risks with anti-clotting drugs. May increase some of the side effects of steroids such as prednisone and hydrocortisone. May blunt effects of the chemotherapy drug cisplatin.

Saw palmetto
Possible benefits: May promote urine flow and reduce symptoms associated with an enlarged prostate (though recent studies have shown results no better than with placebo, according to the NCCIH).
Possible drug interactions: May increase bleeding risks with anti-clotting drugs. Interferes with enzymes affecting the metabolism of other drugs.

St. John’s wort
Possible benefits: May reduce depression (studies are mixed). May reduce symptoms of premenstrual syndrome and chronic fatigue syndrome.
Possible drug interactions: Can reduce blood levels and effectiveness of many drugs, including birth control pills, cholesterol-lowering statins and some anti-HIV drugs. Can increase side effects of some antidepressants and cause increased sedation when combined with alcohol.