You can walk into a drugstore and buy any supplement you’d like. But that doesn’t mean taking it would be a good idea for you—even if your buddy swears by it.
Just like prescription meds, a supplement that provides health benefits to one guy can prove ineffective or even dangerous for another, says Matilde Parente, M.D., author of Healing Ways: An Integrative Sourcebook.
Everything from your overall health to your prescription meds to the way your body breaks down drugs can change the way a supplement works for you, says Bill Gurley, Ph.D., a chemistry professor at the University of Arkansas for Medical Sciences.
So talk to your doctor before popping any kind, Dr. Parente advises. He or she will likely steer you clear of high-risk combos like these below, as well as warn you of other potential dangers based on your health and lifestyle.
1. St. John’s Wort
The problem: You take blood thinners, heartburn meds, or antidepressants
Though it’s shown some benefit in relieving depression, St. John’s Wort poses a high risk of interacting with many prescription medications, Gurley says. The herb contains a unique compound called hyperforin, which boosts the production of drug-digesting enzymes.
As a result, many common medications—including blood thinners like Coumadin or heartburn drugs like Prilosec—break down before they have time to do their job in the body.
“It essentially renders these drugs ineffective,” Gurley says.
What’s more, St. John’s Wort also boosts a brain chemical called serotonin, the same one increased by many prescription antidepressants.
Pairing them won’t lift your mood more quickly—in fact, you could end up with potentially life-threateningly high levels, a serious condition called serotonin syndrome. Early warning signs including agitation, sweating, spasms, and uncoordinated movements.
2. Ginkgo biloba
The problem: You have diabetes
A wide body of research suggests the herb gingko biloba can ease anxiety, boost mental function, and even protect against dementia.
But the supplement also may change the way your body produces or responds to the hormone insulin, possibly because of compounds called ginkgolides.
If you have diabetes, your doctor closely calibrates the dose of drugs like metformin and insulin to reach a specific blood-sugar target. Ginkgo can interfere, leaving your levels too low or too high.
If you do take ginkgo, limit yourself to 240 milligrams per day. Few drug interactions have been shown at this dosage, Gurley says. And work with your doctor to monitor your blood sugar and adjust your medications as needed.
The problem: You take erectile dysfunction or allergy meds
Many people take this herb to treat colds or other respiratory complains, including stuffy noses and hay fever.
But its compounds berberine and hydrastine produce the opposite genetic effect as St. John’s Wort: Goldenseal reduces the production of certain enzymes your body needs to break down some kinds of drugs, Gurley says.
For instance, you need to have enough of these specific enzymes to break down drugs like Viagra and Allegra properly. With goldenseal messing with that ratio, the meds stay potent in your system for longer, which increases your risk of side effects.