Never mix these 5 supplements together: Doctors warn of dangerous nutrient combos that can harm your body

Many of us pop supplements as if they were harmless little sweets. A multivitamin with breakfast, a bit of iron at lunch, and perhaps a magnesium capsule before bed “for better sleep”. Sound familiar? The trouble is, research shows not all supplements get along with each other. Some combinations can reduce absorption, while others may even increase health risks if taken carelessly.

If you’re building your supplement routine, it’s worth knowing which pairings experts advise against. Here are some common mixes you may want to separate.

Calcium + Iron

Both calcium and iron are essential minerals, but studies suggest they compete for absorption in the gut. Think of it as two people trying to squeeze through the same narrow doorway, neither gets through properly.

If you’re taking calcium for bone health and iron for energy or anaemia, avoid taking them at the same time. Spacing them out by at least two hours makes a difference. Research also shows iron is absorbed best on an empty stomach with vitamin C, such as a glass of orange juice.

Magnesium + Calcium

Both minerals support bone and muscle function, but excessive calcium can interfere with magnesium absorption. Nutrition research often recommends balance: some sources suggest a 2:1 calcium-to-magnesium ratio, though individual needs vary.

A practical approach is to take calcium during the day and magnesium at night, which also supports relaxation and sleep.

Zinc + Copper

Long-term zinc supplementation is popular for immunity and skin health, but studies show high doses may reduce copper levels. Copper is vital for blood and brain function, and deficiency can creep up unnoticed.

Nutrition guidelines suggest roughly 1–2 mg of copper for every 15 mg of zinc to keep things balanced. If you’re taking zinc for months at a time, consider whether you’re also getting enough copper.

Fish Oil + Ginkgo Biloba

Fish oil supports heart health, and ginkgo biloba is often used for memory and focus. However, both can thin the blood. Research suggests combining them may increase the risk of bruising or bleeding, particularly if you’re also on blood-thinning medicines.

If you’re taking therapeutic doses of either, it’s best to check with a healthcare professional before doubling up.

Iron + Tea or Coffee

That morning iron pill with your tea or coffee may not be working as well as you think. Studies show tannins and caffeine can inhibit iron absorption, especially from supplements.

To get the most benefit, take iron on an empty stomach with vitamin C and wait at least an hour before having your tea or coffee. Black tea appears to have the strongest blocking effect, but green tea and coffee can also interfere.

Vitamin D + Magnesium

Unlike some clashes, this one is more of a missed opportunity. Research highlights that magnesium is needed to activate vitamin D. Without adequate magnesium, your vitamin D supplement may not work as effectively.

Ensuring you get enough magnesium, through foods such as leafy greens, nuts and seeds, or a supplement, can help your vitamin D do its job.

Vitamin K + Blood Thinners

Vitamin K supports healthy clotting, but this is exactly why experts warn against supplementing it while on blood-thinning medication such as warfarin. Studies show sudden increases in vitamin K intake can interfere with medication doses, increasing the risk of clotting or stroke.

Leafy greens naturally contain vitamin K and don’t need to be avoided altogether, but consistency, not fluctuation, is key. Supplements, however, should only be considered under medical advice.

Melatonin + Other Sleep Aids

Melatonin is a popular sleep aid, but mixing it with other calming supplements such as magnesium glycinate, valerian root or L-theanine may cause next-day grogginess. Research on combinations is limited, but reports suggest excess drowsiness is a common effect.

It’s wise to test supplements separately before stacking them, and to keep dosages modest.

Multivitamins + Extra Supplements

Multivitamins already pack a wide range of nutrients. Adding individual supplements on top, such as extra vitamin A, iron or B-complex, can lead to unintentional overdosing.

Fat-soluble vitamins (A, D, E and K) are of particular concern, since they build up in the body rather than flushing out. Over time, excess levels can become harmful. Checking labels and tracking your intake is a simple safeguard.

So, what’s the safe approach?

Supplements can be valuable tools for supporting health, but more is not always better. Studies consistently show that timing, balance and dosage matter just as much as the nutrients themselves.

Think of your supplements like a team: when chosen wisely, they work together to keep you strong. But the wrong mix can cause confusion and side effects. A sensible first step is to get your levels checked with a blood test, then design a supplement plan that meets your personal needs.

After all, good health isn’t about how many capsules you take, it’s about taking the right ones, in the right way.