Vitamin and supplement interactions to know
A number of combinations of vitamins and supplements should be avoided. “Calcium and iron compete with each other for absorption, so they should not be taken at the same time,” said Thomas. “Also, iron and calcium can interfere with the absorption of thyroid hormone replacement medications like levothyroxine. Vitamin K can counteract blood thinners such as warfarin if taken in high doses. Plus, St. John’s wort can also interfere with antidepressants.”
Combining vitamins may also be a hazard, such as “multiple preparations with B vitamins, vitamin D, and vitamin A,” said Reuter. She also noted that additional combinations to avoid include “multiple supplements with ingredients that affect bleeding and/or blood clotting, multiple supplements that have stimulating effects like preparations for focus, alertness, weight loss, and energy, and drug-supplement-herb combos.”
Long-term treatments may also be negatively affected by herbal supplements. A 2025 study found that older patients undergoing cancer treatment experienced adverse reactions while taking red yeast rice, red coneflower, turmeric, and cannabis.
The bottom line
If you’re already taking supplements, fill up a bag with your bottles and bring them to your healthcare provider. “Tell your doctor what you’re taking,” said Dr. Fontana. “Patients are often reluctant to reveal the supplements they’re taking because they think their doctor will think they don’t trust the care they’re being given. Be honest and open. Doctors just want to help you; we’re not going to judge you.” Transparency means safety when it comes to supplements—the more your doctor helps you, the better your health will be.
Dietary supplements are products intended to supplement the diet. They are not medicines and are not intended to treat, diagnose, mitigate, prevent, or cure diseases. Be cautious about taking dietary supplements if you are pregnant or nursing. Also, be careful about giving supplements to a child, unless recommended by their healthcare provider.
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