Popping a daily vitamin is common practice—as many as 75% of Americans may do so. But some people aren’t just stopping at one vitamin: They’re supplement stacking, combining several vitamins and minerals at once.
Still, not all vitamins and supplements are designed to go together, especially in a more concentrated forms. With that, it’s important to know what vitamins should not be taken together.
Meet the experts: Sonya Angelone, R.D.N., is a nutritionist and registered dietitian based in San Francisco, California; Bruce Ruck, Pharm.D., is a toxicologist, adjunct professor at the Rutgers College of Pharmacy, and managing director at the New Jersey Poison Information and Education System; Zachary Mulvihill, M.D., is an integrative medicine physician at Weill Cornell Medicine
While supplements may seem like a harmless thing, that’s not always the case, according to Sonya Angelone, R.D.N., a nutritionist and registered dietitian based in San Francisco, California. “They can talk to your DNA to affect how cells work, they can influence immune function, or they can contribute to an excess of a nutrient which can create a toxicity if not excreted properly,” she says. If you pair the wrong supplements together, Angelone says they can work against each other or even lead to uncomfortable side effects for you.
Plus, vitamins and supplements are part of a largely unregulated industry, making it crucial to know what you’re taking, says Bruce Ruck, Pharm.D., a toxicologist, adjunct professor at the Rutgers College of Pharmacy, and managing director at the New Jersey Poison Information and Education System.
Being harmed by vitamins and vitamin interactions is somewhat rare, “but it’s common enough that it’s a real concern and it does come up for me clinically,” says Zachary Mulvihill, M.D., integrative medicine physician at Weill Cornell Medicine.
All of that said, experts agree that some people can benefit from taking vitamins and even several at once—it’s just important to make sure they don’t work against each other and to get your doctor’s buy-in.
What Vitamins Should Not Be Taken Together?
If you’re planning to put yourself on a vitamin regimen, Ruck says it’s always a good idea to talk to a healthcare provider first. “Talk to your physician, a nurse, or pharmacist—check with somebody,” he says. They can do bloodwork to determine if you have any deficiencies and take into account any other health conditions you may have or medications you are on.
While this is not an exhaustive list, having your own baseline knowledge never hurts. And, while some of these are vitamins or minerals, others are herbal supplements that may have strong interactions.
Zinc and calcium
Zinc is a mineral that helps support the function of your immune system and metabolism, while calcium helps to build and maintain bones and teeth, and supports muscle function and nerve transmission.
While each has its own role in the body, they shouldn’t be taken together. “Minerals usually compete for absorption from the intestines,” Angelone says. “Too much of one mineral will block the absorption of the other one.” Basically, if you take both of these at once, “you will absorb less of both,” Angelone explains.
Iron and zinc
Iron is used in the body to make hemoglobin, a protein found in red blood cells. It’s also required for growth and development, according to the National Institutes of Health (NIH). Healthcare providers may recommend taking iron if you have anemia, a condition where the body doesn’t have enough healthy red blood cells. But taking iron with zinc isn’t recommended. “They compete for a common absorptive pathway,” Dr. Mulvihill says.
As a result, “that can decrease the absorption of either the iron, the zinc, or both,” Ruck says.
Iron and calcium
These two also don’t play well together. “High levels of calcium seem to decrease the absorption of iron,” Dr. Mulvihill says. When these two are taken together, iron is usually absorbed less, Angelone explains. Iron deficiency could potentially occur in those at risk for anemia, she says.
Magnesium and zinc
Magnesium plays a role in several areas of the body, including regulating nerve function as well as calcium and blood sugar levels. But magnesium and zinc compete for the same pathways in the body, making it tough to absorb both at once, Angelone says.
Magnesium is usually sacrificed when this happens, Dr. Mulvihill says. “High doses of zinc can increase magnesium deficiency,” he points out.
St. John’s wort and serotonergic supplements
St. John’s wort is an herb with a wide range of uses, including for depression, stomach ulcers, colds, and wound healing, according to the National Center for Complementary and Integrative Health (NCCIH). Serotonergic supplements are designed to increase levels of the feel-good hormone, serotonin, in the body to support good mood and sleep. These may include vitamin B6 and folate, among others.
Experts are especially wary of taking St. John’s wort without the guidance of a healthcare provider. “This is a tricky supplement,” Angelone says. Taking St. John’s wort with a serotonergic supplement can cause excessive serotonin in your brain and lead to something called serotonin syndrome, which is a potentially life-threatening reaction, she says.
Ginkgo biloba and high-dose fish oil
Ginkgo biloba is an herbal supplement that may impact blood circulation and cognitive function. Fish oil is typically used to promote heart health and to reduce inflammation in the body.
The big concern with taking these together is that they can impact your blood platelets and increase your risk of bleeding, Ruck says. It’s also not a good idea to take either of these on top of a blood thinning prescription medication, Dr. Mulvihill says. “Anyone on a blood thinner should be especially careful,” he says.
St. John’s wort and ginseng
Ginseng is an herb with a range of uses, including boosting energy and stabilizing blood sugar. “Taken together, they can affect energy and mood by overstimulating the central nervous system,” Angelone says.
Dr. Mulvihill also stresses the importance of being cautious with St. John’s wort. “I do not use it with any of my patients because it has such significant risks for interactions,” he says. Birth control is one of the medications this supplement may interact with, Dr. Mulvihill says.
Which Vitamins Are Better When Taken Together?
While some vitamins and minerals shouldn’t be taken together, there are certain combinations that may be better when they’re used as a pair. Again, though, talk to your doctor before starting any new vitamins or supplements.
Vitamin C and iron
Some iron supplements will include vitamin C for a very specific reason. “Vitamin C enhances the absorption of non-heme iron from the stomach,” Angelone says. (Non-heme iron is a type of iron from plant foods.)
Magnesium and vitamin D
Magnesium enhances vitamin D metabolism, Angelone says. “But [it] doesn’t need to be taken at the same time since it isn’t involved with the actual absorption of vitamin D,” she explains. “It should just be taken in the same day.”
Vitamin D, calcium, and vitamin K2
If you’re taking calcium, Angelone suggests looking for a supplement that also contains vitamins D and K2, or taking them as well. “Vitamin D enhances calcium absorption from the intestines into the blood, so more vitamin D means more calcium in the blood stream,” she says. “In order for calcium to get into the bones, it needs to have an active transport molecule. Vitamin K2 is the only nutrient that activates this protein transporter.”
FAQCan you take other vitamins with a multivitamin?
You can take a vitamin on top of a multivitamin, but experts say it’s crucial to make sure you’re not doubling up on some nutrients. “I’m surprised at how often this comes up,” Dr. Mulvihill says. “Some people will take a multivitamin that has a B complex and also take something for their hair that has B vitamins in it.”
It’s also important to be mindful of potential interactions, Ruck says. “For example, if your multivitamin has iron and you take a separate one with zinc, they may interact,” he says.
Can you take medication at the same time as a vitamin?
It’s a good idea to talk to your healthcare provider about this one. “Some vitamins and minerals can interact with medications, especially antibiotics,” Ruck says. “Taking them together could make your medication ineffective.”
How can you know if you need a vitamin?
The best way to know for sure is to talk to a healthcare provider and undergo testing. “It’s always better to test for a deficiency than to supplement blindly,” Dr. Mulvihill says.
What should you do if you take vitamins that may interact?
If your doctor recommends that you take vitamins that may interact, Ruck says it’s a good idea to separate them by a few hours. That may mean taking one supplement in the morning and the other at lunch.
Bottom line
While supplements are incredibly popular and easily accessible right now, experts say it’s still best to check in with a healthcare provider before diving in. If you’ve already started a vitamin regimen, Dr. Mulvihill suggests that you still talk to a healthcare provider.
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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