Forgetting your multivitamin may not be so bad: One in three Americans puts their health on the line by taking potentially hazardous combinations of prescription medications and dietary supplements, reports a new study from the U.S. Army Research Institute of Environmental Medicine (USARIEM). [Tweet this stat!]

"Many people mistakenly believe that because supplements can be obtained without a prescription, they're safe," says study author Harris Lieberman, Ph.D. But some herbal ingredients can interfere with enzymes that your body uses to break down medications, affecting the potency or effectiveness of other prescriptions, he explains.

So why didn't your doctor warn you? Most people don't think to include fish oil or iron supplements on their "daily medication" list, so your doc may not know the script he's writing could pose a health issue. "It's very important to check with your doctor about taking a supplement on top of medication," Lieberman says.

The combinations to steer clear of (like prescription pills and booze) can be obvious. But others-some seemingly innocent pairings-can be just as hazardous. Here are five.

Multivitamins and Most Serious Meds

Multivitamins contain so many ingredients already, and many brands now offer additional support (like One-a-Day plus DHA or plus immune protection). The more nutrients, the higher the chance something interacts with your prescription medications, says Lieberman. Plus, in over 25 percent of bottles, the vitamins and mineral levels on the label don't match the dosage, according to a 2011 analysis from ConsumerLab. This means you might not be safe from combinations that are only a threat at high doses-like Vitamin K and blood thinners or iron and thyroid medications.

St. John's Wort and Birth Control

The herb that promises to fight depression can also weaken the effect of serious prescriptions like heart and cancer meds, allergy medications, and birth control pills. In addition to reports of unintentional pregnancies while taking the two, an FDA study found 300 milligrams (mg) of St. John's Wort three times a day (similar to the recommended dosage for depression) may alter contraceptive's chemical makeup enough to warrant additional protection.

Vitamin B and Statins

Niacin-better known as vitamin B-is used as a natural remedy for everything from acne to diabetes, but it can harm your muscles if taken with cholesterol-lowering statins. Both vitamin B and statins weaken muscles, which individually just means possible cramps or aches. Together though, the side effect is compounded: One quarter of people taking niacin and statins as part of a 2013 heart study dropped out because of reactions including rashes, indigestion, and muscle problems-29 people developed the muscle fiber condition myopathy.

Decongestants and Blood Pressure Medications

Decongestants, particularly brands with pseudoephedrine (Allegra D and Mucinex D), clear your stuffy nose by constricting blood vessels, lowering the swelling and draining the fluid. But the drugs narrow blood vessels throughout your body too and can raise your blood pressure slightly, which could counteract medication and pose an issue for someone with high blood pressure, says the American Heart Association (AHA). A lot of unsuspecting cold and flu medicines have decongestants in them, the AHA adds, including some favorite brands: Clear Eyes drops, Visine, Afrin, and Sudafed.

Fish Oil and Blood Thinners

Omega-3-packed supplements get (and deserve) praise for heart benefits, but they also thin your blood. While this isn't a rare or worrisome side effect normally, if you're also taking blood thinners (like warfarin or aspirin), you could be increasing your risk of excessive bleeding, according to the Cleveland Clinic. The jury's still out on how much fish oil makes for a harmful combination, but tell your doctor if the supplement is part of your routine. In fact, if you're on a blood thinner, talk to your M.D. about what nutrients to avoid. Many herbs and minerals have natural coagulant effects-even chamomile tea.