Jun 26, 2025
Some Vitamins and Supplements Shouldn’t Be Taken Together, Here’s Why
Vitamin supplements are an easy way to support your health, especially if your diet isn’t always perfect. But taking all vitamins at once doesn’t always work in your favor.
Health Hacks
Written by
Mito Team
There are some combinations of vitamins and supplements that can interfere with how your body absorbs or uses nutrients. Some compete for the same pathways, while others work against each other’s function. That means you could be taking healthy vitamins that never actually reach your cells.
Here’s how to make the most of your nutrition supplements by avoiding combos that cancel each other out.
Nutrients That Compete for Absorption
Iron and Calcium
Iron helps your red blood cells carry oxygen, while calcium strengthens your bones. But these two essential minerals compete for absorption in your digestive tract. That’s why taking them together, like in a multivitamin or meal, reduces the amount your body absorbs, especially iron.
A study showed calcium supplements cut iron absorption by up to 62% when taken with food. While its effect is real, it’s usually short-lived and may not impact blood iron levels long-term.
Better approach: Take calcium and iron at least two hours apart. And if you’re low on iron, try pairing it with vitamin C instead to improve absorption.
Iron and Zinc
Iron and zinc are both essential minerals. Iron carries oxygen in the body and gives us energy, while zinc plays a key role in the immune system, healing wounds, and making DNA. These two essential vitamins share the same transport system in your gut. When they are taken in high doses together, they can compete with each other.
It was found in a study that high doses of iron can significantly inhibit zinc absorption, especially when both are taken on an empty stomach. The reverse is also true: too much zinc can reduce the uptake of iron.
What to do: If you need both, consider taking them at different times of day. For example, zinc with breakfast and iron with lunch or dinner. You can also use a balanced supplement that includes carefully calibrated doses of both, often in a zinc-to-iron ratio of 1:1 or 1:2.
Calcium and Magnesium
Calcium and magnesium are two of the most commonly used mineral supplements. They are often bundled together for bone and muscle health. However, when you take them in high doses, they can compete with each other in your gut.
If you have high levels of calcium, it can actually make it harder for your body to absorb magnesium. This is especially important for people who eat a lot of calcium-fortified foods or take calcium supplements.
Tip: Rather than taking both at the same time, try splitting the doses. Take your calcium in the morning and a magnesium supplement at night for sleep and muscle relaxation.
Nutrients That Interfere With Function
Vitamin C and Vitamin B12
Vitamin C is great for boosting your immune system and has strong antioxidant properties. Vitamin B12, on the other hand, helps keep your nerves healthy, makes red blood cells, and is important for DNA synthesis. However, high doses of ascorbic acid (500 mg or more) break down B12 or make it harder for your body to absorb it.
Fix: If you take both, space them out by at least 2 hours. Take B12 in the morning and vitamin C later in the day. This is especially important for people over 50, vegetarians, or anyone at risk of low B12 levels.
Vitamin A and Vitamin E
Both vitamin A and vitamin E are fat-soluble antioxidants. This means your body stores them in fat. Vitamin A is best for good vision, healthy skin, and a strong immune system, while vitamin E protects your cells from oxidative stress.
However, taking over 400 IU of vitamin E per day can mess with vitamin A absorption. This gets worse if you're low on vitamin A or if you take supplements for a long time.
Solution: The best approach is to stick to moderate doses of both vitamins, unless advised by a healthcare professional. Avoid combining high doses of vitamin E with vitamin A supplements unless it's part of a balanced formula.
Vitamin E and Vitamin K
Vitamin E is often taken to support skin and heart health due to its antioxidant effects. Vitamin K, however, is key for blood clotting and managing calcium levels. The issue comes when people take high doses of vitamin E (more than 800 IU per day) for a long time.
Studies show that too much vitamin E can interfere with vitamin K, especially vitamin K1, which messes with blood clotting. This effect is risky for those on blood thinners like warfarin, as it can raise the risk of bleeding.
Best practice: Avoid high-dose vitamin E supplements unless a doctor says it’s okay. For most people, getting vitamin E from food sources like almonds, sunflower seeds, and leafy greens is sufficient and won’t mess with vitamin K.
Nutrients That Need to Stay in Balance
Vitamin D and Vitamin K
Vitamin D helps your body absorb calcium from food and supplements, which is important for bone health and other functions. Vitamin K, especially K2 (menaquinone), plays a role by
activating proteins that direct calcium into your bones and teeth instead of building up in your arteries.
Recent findings suggest that taking vitamin D without enough K2 might lead to vascular calcification, especially for those already taking calcium supplements. Basically, while vitamin D helps raise your calcium levels in the blood, vitamin K2 makes sure that calcium goes to your bones and teeth, instead of building up in your arteries.
Combining vitamin D3 (2000 IU) with vitamin K2 (180 mcg) improved bone mineral density more effectively than vitamin D alone in postmenopausal women. Other studies suggest the vitamin D and K combo may help with arterial stiffness, though results on heart health remain mixed.
Tip: Look for a D3 + K2 supplement for better results, especially if you’re taking calcium. Fermented foods like natto and hard cheeses are great natural sources of K2.
Zinc and Copper
Zinc is important for your immune system, healing, and enzyme function. Copper helps with making your red blood cells, strengthening connective tissues, and providing antioxidant support. The catch is they share a transporter in the gut, so if you take too much zinc, it locks copper absorption.
This has been shown in many clinical and laboratory studies. For example, research found that taking 50 mg of zinc daily for over 10 weeks reduced copper levels in the blood. It even led to signs of copper deficiency in some participants, like anemia and low white blood cell counts.
Watch the ratio: If you're taking zinc for a long time, make sure to get some copper too, ideally in a ratio of 10:1 or 15:1 (zinc to copper). Some multivitamins or combo supplements are designed with this balance in mind.
Supplement Smarter, Not Just More
You don’t need to avoid any of these health vitamins altogether—you just need to take them at the right time. Being strategic with your vitamins and supplements can help you get real results, avoid imbalances, and make the most of your effort.
At Mito Health, we can help you personalize your supplement routine based on the results of your biomarker blood test. Because when it comes to all vitamins, timing, balance, and dosage matter.