Recently, I had a hunch my iron was low. (You too? Here are some signs and symptoms to look for.) I consulted with my dietitian, who pointed me to an iron supplement that would work well for me. She also made a note to let me know that iron is best taken away from calcium supplementation and/or the consumption of higher-calcium foods such as dairy products like cheese, milk, and yogurt.

This is because iron and calcium compete for absorption in the body. (Who knew!) Nothing bad would happen if I took iron with calcium, but the calcium would just lower the iron absorption in my body a bit. Crazy stuff, right?

It led me to question, “What vitamins don’t mix” and “What vitamins should you not mix?” Here’s what RDNs had to say on the subject, including what vitamins you should take together and when.

What vitamins should you not mix?

Okay, so what vitamins should you not mix? Well, there are only a few, but the interactions mentioned below occur due to the way these minerals use similar absorption pathways in the digestive tract, explains Mary Sabat, RD, founder of Body Designs by Mary. “When taken together in high amounts, they can compete for uptake, potentially leading to imbalances and reduced effectiveness,” she says.

So what are they? Read on.

1. Calcium and magnesium

Although they’re often combined in supplements, large amounts of calcium can inhibit the absorption of magnesium, explains Sabat.

2. Iron and calcium

What vitamins don’t mix? Well, you already know this one — iron and calcium. “Avoid taking iron and calcium supplements together as they can compete for absorption in the digestive tract,” says Sabat. “Calcium can interfere with the absorption of nonheme iron (the type of iron found in plant-based sources and supplements), reducing its uptake by the body.”

Iron is also best absorbed when taken in the presence of vitamin C, so feel free to double up those two together or take iron with vitamin C-packed foods like your morning glass of freshly squeezed orange juice, that half of a grapefruit at lunch, or those red bell peppers at dinner.

3. Iron and zinc

Okay, iron, I see you. You clearly don’t play nice with others in the sandbox. “Similar to the interaction between iron and calcium, iron and zinc can also compete for absorption,” notes Sabat. “High doses of zinc, especially when taken as a supplement, can hinder the absorption of iron. Therefore, it is advisable to separate the intake of these two minerals if you are taking them as supplements.”

Remember, it’s best to take iron with vitamin C-rich foods, so stick to that cadence — and if you need some additional zinc, just make sure you take it a few hours before or after that iron supplement.

4. Calcium, magnesium, and zinc

Now zinc is front and center of what vitamins don’t mix. Another combo you may want to avoid is calcium, magnesium, and zinc. Calcium likes to be the star of the show and definitely has some main character energy, which is why it can affect how other vitamins and minerals are absorbed in the body. That includes iron (as noted above), magnesium, and zinc. 

“Calcium, magnesium, and zinc can all interact with each other if taken together in high amounts,” explains Sabat. “These minerals can compete for absorption, potentially reducing the effectiveness of each mineral.”

Sabat says if you’re taking these three minerals individually, consider spacing them out throughout the day to ensure optimal absorption — but if you’re getting them all in a multivitamin, don’t worry. Multivitamins usually contain smaller doses of these minerals, so you’re in the clear.

5. Copper, iron, or phosphorus with zinc

Again with the zinc? (A total troublemaker, if you ask me.) Besides avoiding zinc with calcium, also make sure you’re not mixing it with copper, iron, or phosphorus supplements.

“High doses of zinc can interfere with the absorption of other minerals like copper, iron, and phosphorus as excessive zinc intake can lead to imbalances in these minerals, which could affect overall health,” mentions Sabat. Instead, space them out a couple of hours apart.

Oh, and the same can be said about copper and zinc all on their own: “Large amounts of zinc can reduce copper absorption, and vice versa,” notes Sabat.

As a general rule of thumb, larger amounts of minerals taken at the same time as other minerals will reduce the overall absorption of them all — and as much as we spend on supplements like minerals and vitamins, we want the most bang for our buck.

What vitamins should you take together?

1. B vitamins

B vitamins are usually recognized as a group, which means they roll together pretty well. Sabat says B vitamins, such as B6, B12, and folate, often work synergistically. This means they often work together in the body as a team, which is why they can be taken together.

“They play vital roles in energy metabolism and various biochemical processes in the body,” explains Sabat. “Taking a B-complex supplement can help ensure you get adequate amounts of all B vitamins.”

2. Iron and vitamin C

As noted above, vitamins that can be taken together also include iron and vitamin C. This is because vitamin C strongly increases the absorption of nonheme iron, which is iron from plant-based sources and supplements. Consuming vitamin C-rich foods or supplements alongside iron sources can significantly improve iron absorption, notes Sabat.

3. Vitamins A, D, E, and K

Vitamins A, D, E, and K and omega-3s like DHA absorb best with a meal or snack that contains some fat because, not a hot take here folks, these vitamins are fat soluble, meaning the body absorbs them best in the presence of fat.

“These fat-soluble vitamins work together to support various aspects of health, including antioxidant protection, bone health, and immune function,” says Sabat. “They are often found in well-balanced multivitamins.”

If you’re taking one, or all, of these vitamins, pair them up with some avocado toast, peanut butter, or a fatty piece of fish like salmon. (Need some new ideas and ways to get creative with peanut butter? We got you, boo.)

4. Vitamin C and zinc

According to a 2020 article published in the journal Nutrients, “The prevalence of inadequate nutrient intakes that support the immune system (vitamin A, C, D, E) remain substantial and some are higher (vitamins C, D, and zinc) than previously reported.”

Translation: “Vitamin C can enhance the absorption of zinc, making it a good idea to take these two together, especially during cold and flu seasons,” states Sabat.

If you’re working on that immune health, feel free to take vitamin C and zinc together. (Finally! Something zinc plays well with.)

What about multivitamins?

Okay, so if there are answers to the question, “What vitamins don’t mix,” what about multivitamins? Everything is combined in one pill there, so are they still safe to take?

Multivitamins contain a mix of vitamins and minerals that can be beneficial for individuals with dietary deficiencies or specific needs. However, there are a few considerations, notes Sabat.

Individual needs: Not everyone requires a multivitamin as individual nutrient needs vary based on age, diet, gender, health status, and other factors. It’s a good idea to consult a healthcare professional before starting any supplement regimen.

Quality: Choose a reputable brand with quality ingredients and balanced formulations. Avoid megadoses of vitamins and minerals as excessive intake can lead to imbalances and potential health risks.

Timing: Taking a multivitamin with food can enhance absorption as many vitamins are fat-soluble and need dietary fat for optimal uptake. Dividing the dose throughout the day can also help avoid overwhelming the body’s absorption mechanisms and promote better utilization.

The bottom line on what vitamins don’t mix

If you were ever wondering what vitamins you should not mix, here’s a list of some general rules on what vitamins don’t mix and also what vitamins do mix.

“While there are certain vitamin and mineral combinations to avoid due to absorption conflicts, taking some vitamins and minerals together can enhance their effectiveness,” notes Sabat. “When considering multivitamins, choose wisely and consider your individual nutritional needs.”

Obviously, one of the best ways to get in all of your daily vitamins and minerals is through real food sources — but let’s be real, that can’t always happen, which is why there are supplements. Just be sure to consult your dietician or primary care physician when starting a new supplement.