Dietitians Warn You Should Never Take These Vitamins Together
Hannah Chenoweth, Laura Iu, R.D., C.D.N.
Sat, May 3, 2025 at 6:23 AM CDT
5 Dangerous Vitamin Combinations to Avoid Tanja Ivanova - Getty Images
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Taking all your vitamins and minerals together seems like an easy way to add some wellness to your regimen, right? While “all-at-once” is a convenient approach, it isn’t necessarily optimal, according to doctors and nutritionists.
Some vitamins and minerals can counteract each other, reducing their effectiveness and potentially causing side effects like nausea, diarrhea and constipation. To get the full benefits from your supplements (and let’s be honest, your money), it’s important to understand which combinations to avoid.
“Many people don’t realize how much timing matters when it comes to supplements,” says Amy Margulies, R.D.N., a registered dietician nutritionist and owner of The Rebellius RD. “Some are better absorbed with food, while others are best absorbed on an empty stomach. Other combinations can actually interfere with each other or with medications.”
In other words, mixing certain supplements can actually have the opposite effect of improving your health. In more extreme cases, long-term nutritional deficiencies and vitamin toxicity can occur, says Brynna Connor, M.D., a board-certified family medicine physician and the Healthcare Ambassador for Northwestpharmacy.com,
That doesn’t mean you should be scared to take your supplements! Just do what you can to avoid the vitamin and mineral combinations below. That way, you can protect your health and get the benefits you're looking for from supplements.
Don't mix: Iron and calcium
Calcium interferes with iron’s absorption in the gut, making this combo a no-go. If you’re low in both levels, you can still take both calcium and iron, as long as you space them out safely.
What to do: Take iron at least two hours before or after calcium-rich foods (like dairy products, leafy greens or canned fish with bones) or supplements. Calcium is best absorbed when taken in smaller doses (500 milligrams or less) spread out during the day, says Lamees Hamdan, M.D., an integrative medical doctor. While calcium citrate can be taken with or without food, it’s best to take calcium carbonate with food, since it relies on stomach acid for optimal absorption.
Don't mix: Iron and zinc
Similar to iron and calcium, iron and zinc don’t get along so well. Research shows they actually compete for the same absorption pathways in our gut. “If you’re taking a high-dose iron supplement and zinc at the same time, especially on an empty stomach, there’s a good chance your body won’t absorb the zinc as well,” says Margulies.
What to do: “For optimal absorption, it’s generally best to take iron with food (just not with calcium) and to separate it from zinc,” says Margulies. Because certain foods can interfere with zinc’s absorption, most doctors advise taking it on an empty stomach. However, if zinc upsets your stomach, it's perfectly fine to take it with a small snack. Just stay away from taking it with foods rich in calcium, iron or fiber, says Margulies.
Don't mix: Copper and zinc
If you’re taking a zinc supplement, it’s important to know that high doses of zinc can reduce copper absorption. This can lead to a potential copper deficiency over time. Eating copper-rich foods (such as shellfish, seeds and nuts and organ meats) may balance out this effect, but it’s a good idea to stay on top of it and get your levels checked regularly.
What to do: If you do need to take both copper and zinc, separate them by at least two hours. Both are safe to take on an empty stomach. “You can take zinc in the morning before breakfast and copper before lunch or dinner, provided you have not consumed a meal or snack within at least a few hours,” says Margulies.
Don't mix: Calcium and magnesium
Calcium and magnesium compete for absorption in the gastrointestinal system, which means taking them together in high doses can cause stomach discomfort or issues such as bloating and/or diarrhea, says Dr. Connor.
What to do: Take calcium and magnesium separately, with — you guessed it — at least two hours in between. Calcium is better suited for breakfast (since calcium carbonate should be taken with food), whereas magnesium’s relaxation-promoting benefits make it a perfect fit for your nighttime routine.
Don't mix: Vitamin B12 and Vitamin C
Because vitamin C creates an acidic environment, research shows large doses can potentially degrade vitamin B12 before it gets the chance to work its magic.
What to do: Dr. Connor recommends taking them in alphabetical order: Take vitamin B12 first and hold off on vitamin C for at least two hours. You can pair vitamin B12 with a light breakfast — it’s also fine to take it on an empty stomach.
Ideal vitamin pairs
There are also plenty of complementary vitamins and minerals that boost each other’s effects. While experts agree that you should prioritize meeting your nutritional needs through your diet first and foremost, here are some dynamic duos that you can also take in supplement form:
Vitamin C and iron: Vitamin C enhances the absorption of iron, especially the kind found in plant-based sources like beans and lentils. “This combo is a great one-two punch for preventing iron deficiency and keeping energy levels up,” says Margulies.
Vitamin D and vitamin K12: These vitamins work synergistically to optimize calcium delivery and support bone health, says Dr. Hamdan.
Vitamin A and iron: Research shows this pairing is even more powerful together, especially for people dealing with anemia.
What about multivitamins?
If certain vitamins and minerals inhibit each other’s absorption, you might wonder about the safety of multivitamin supplements. “Multivitamins are specifically formulated with smaller, balanced doses that aren’t competing with each other,” explains Margulies.
Organics Multivitamin for Women
Multivitamin Tablets
Essential for Women Multivitamin
The bottom line
Before starting any new supplement — even an all-natural one — always consult with your healthcare provider. It’s the safest way to avoid potential contraindications with your current medications, especially blood pressure meds, thyroid meds, and anticoagulants, says Dr. Connor. “It’s always best to stick to a routine when it comes to taking your vitamins and minerals,” she says. “Once you know how to space them out safely, taking them at the same time each day can improve absorption and consistency.”
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