Can My Vitamin Make My Medicine Not Work?

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Supplements are big business in the United States. According to survey data from the American Osteopathic Association in 2019, 86% of American adults take a least one vitamin or another supplement.

While some supplements may have value for health and wellness (and even treating certain medical conditions), they are not without risks. One of the biggest concerns is adverse interactions between certain medications and supplements.

"There are multiple mechanisms by which a supplement can interact with a medication," says Yaiseli Figueredo, PharmD, a clinical pharmacist with the University of Miami Health System. "Some supplements can interact with how our bodies metabolize and excrete medications, while others can increase or decrease the effects of medications."

Other supplements interfere with the absorption capacity of medications and reduce the healing effect.

The list of potential drug and supplement interactions is quite lengthy.

Here are just a few of the common ones:

  • Vitamin K (taken alone or in multivitamins) can prevent blood-thinning medications such as warfarin from working correctly.
  • St. John's wort impacts the effect of many different medications, including SSRIs (selective serotonin uptake inhibitors).
  • Potassium supplements can be dangerous when taken with some heart medications, such as ACE inhibitors or angiotensin receptor blockers.
  • Calcium, aluminum, and iron supplements can reduce the absorption of some antibiotics.
  • Garlic, green tea, and echinacea supplements can reduce the impact or increase side effects when taken with some chemotherapy drugs.

Since these interactions are so multiple and varied, Dr. Figueredo says that the best way to prevent a potential complication is through research and close communication between patients and their doctors or pharmacists.

"I always encourage patients to do their homework and educate themselves on the potential risks of interactions between a supplement they wish to start and their prescription medications," she says. "I highly recommend all patients to reach out to their local pharmacist, and they will be able to provide detailed reports on drug-supplement interaction information."

Always tell your doctor exactly what supplements you take.

If a doctor prescribes a new medication, be upfront about any supplements you are - or are considering - taking so the doctor can let you know if there are any related risks.

"Always be transparent with health care providers regarding all supplements patients consume," says Dr. Figueredo. "Supplements and herbal remedies are perceived to be benign and natural by patients and therefore assumed to be exempt from being mentioned to their health care provider.

"Unfortunately, some supplements can interact with how our bodies metabolize and excrete medications."

While some appear safe and effective, the reality is that supplements are not as closely regulated as medications. Your provider may have recommendations about which supplement you should take (if any) and at what dosage.

"Ultimately, individuals should consult and disclose to their medical team any supplements they take," says Federika Garcia Muchacho, a senior clinical dietitian with the Sylvester Comprehensive Cancer Center.

"I also recommend they look for supplements with the USP Verified Mark, which ensures supplements have been third-party tested to comply with FDA guidelines. Consumer Lab is another consumer-friendly website. They independently test vitamins, minerals, herbs, and other nutritional supplements for accuracy and quality."


Wyatt Myers is contributing health writer for UHealth.