American supplement shelves keep growing, but the biggest risks are concentrated in a small handful of products. A widely cited Mayo Clinic study found that just five herbal supplements, garlic, valerian, kava, ginkgo and St. John's wort, accounted for 68 percent of potentially clinically significant interactions with prescription medications.
Many patients who combine the two never tell their doctor, according to the review in American Family Physician.
Assume "natural" does not mean safe
The Food and Drug Administration is not authorized to approve dietary supplements for safety and effectiveness before they reach store shelves. Manufacturers, not federal reviewers, decide what goes into the bottle.
That is a good reason to bring every vitamin, herb and powder to your next appointment. Interactions your pharmacist can flag on a prescription printout do not always show up on a supplement label.
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The most dangerous herb on the shelf
St. John's wort, promoted for low mood, tops most drug-interaction lists. Combining it with SSRIs, SNRIs or MAO inhibitors can trigger serotonin syndrome, a potentially life-threatening spike in the brain chemical serotonin.
Symptoms can include agitation, rapid heartbeat, elevated blood pressure, hallucinations and fever, according to the National Center for Complementary and Integrative Health. The agency also lists interactions with birth control pills, warfarin, cyclosporine, digoxin and several HIV and cancer drugs.
Kava and the liver
Kava, sold for anxiety and stress, carries a separate danger. The FDA issued a consumer advisory in 2002 linking kava products to severe liver injury, including hepatitis, cirrhosis and liver failure, after cases required liver transplantation. Anyone with existing liver disease, or on medications that stress the liver, should be especially cautious.
Watch vitamin K if you take warfarin
Warfarin, sold as Jantoven, works by blocking vitamin K. Sudden swings in vitamin K intake, from leafy greens or a new multivitamin, can make the medicine less effective or more risky, Mayo Clinic notes. The advice is not to strip vitamin K from your diet but to keep intake steady day to day.
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Ginkgo and garlic supplements can also raise bleeding risk on warfarin. Bring them up with your doctor before adding either.
Fat-soluble vitamins build up
Vitamins A, D, E and K dissolve in fat and are stored in the liver and fatty tissue rather than flushed out in urine. Long-term high doses can accumulate to toxic levels.
Multivitamins, protein powders and fortified foods already contain many of these nutrients. Adding a single-vitamin pill on top can quietly push daily intake past safe limits.
Timing matters, too
Calcium supplements can inhibit iron absorption when the two are taken together, according to the NIH Office of Dietary Supplements. Anyone taking both for a diagnosed deficiency should separate the doses by several hours.
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Never swap a prescription for a supplement
The most serious mistake, both Mayo Clinic and NCCIH stress, is using an herbal product to replace a prescribed medication. The federal agency warns specifically against using St. John's wort to replace conventional care or to delay seeing a health care provider about a mental health problem. Anyone considering stopping or switching a prescription should raise it with their doctor first.
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