The lowdown on nutritional supplements: Never take these together and avoid side effects, vitamin overdoses

Many of us who take supplements will often swallow them all at once because it feels like the easiest way to remember them. How often have you bought a supplement with good intentions, only to realise weeks later that the bottle is still almost full?

It is important to know that taking certain supplements together can be a problem. Some combinations interfere with each other’s absorption, while others amplify effects in the body in ways that may not be safe.

When we talk about “supplements you should never take together,” we’re usually referring either to an increased risk of side effects or to combinations that mean you don’t get the benefits you’re expecting.

When it comes to minerals, many share common transport pathways in the gut. At higher doses they compete with one another, which can hinder absorption. This is most relevant if you are trying to treat a specific mineral deficiency, rather than just taking a low‑dose multivitamin.

Calcium and magnesium are both very popular supplements, but the more important interaction is between calcium and iron. Calcium can significantly reduce the absorption of non‑haem iron and can also affect uptake of some other minerals when taken together as supplements. If you’re on iron for anaemia, it’s wise to take it away from calcium tablets and calcium‑rich antacids by at least two hours.

Iron and zinc also compete. They use similar intestinal transporters, so when you take high doses together, the absorption of one or both is reduced, potentially prolonging deficiency. If you are deficient and need both, it is better to stagger the doses across the day rather than combining them in one hit.

Zinc supplementation is very popular for immunity and skin health. However, chronic high zinc intake can induce copper deficiency and lead to anaemia and neuropathy. A common, often overlooked issue is taking a separate zinc supplement on top of a multivitamin that already contains zinc, so always read labels carefully. In the clinic, I will prioritise the most urgent deficiency and dose that nutrient away from other competing supplements.

Some vitamins can also affect each other’s stability when combined at high doses. Large doses of vitamin C, which many people take when they are fighting an illness, can reduce the availability of vitamin B12 in the gut if taken at the same time. To be on the safe side, I generally recommend keeping large doses of vitamin C and therapeutic B12 separate by at least two to three hours.

Taking a lot of the fat‑soluble vitamins A, D, E and K can become harmful over time. People using multiple products such as “hair, skin and nails” formulas, multivitamins and “immune support” blends may inadvertently double up on these nutrients.

For most healthy adults, a single daily multivitamin isn’t usually the problem, issues arise when several high‑dose products are stacked without tracking the total intake.

Herbal supplements and nutraceuticals are especially important to review around surgery and in relation to blood clotting. Omega‑3 supplements can prolong bleeding time, and when combined with ginkgo biloba, garlic extracts or other anti platelet herbs, they can raise bleeding risk further. This is particularly concerning for people also taking aspirin, warfarin or direct oral anticoagulants. A common peri‑operative recommendation is to stop non‑essential supplements about 2 weeks before an operation.

For anyone on blood thinners, most guidelines recommend avoiding non‑essential herbal supplements altogether or introducing them only with close monitoring.

Some botanicals also alter liver enzymes and drug transporters, changing how your body handles medications (and sometimes other supplements). St John’s wort, for example, is linked to lowering blood levels of many drugs, including antidepressants, oral contraceptives, HIV medicines, immunosuppressants and some cardiovascular drugs. The result can be loss of therapeutic effect, so it should never be added casually on top of a prescription regimen.

Always keep an up‑to‑date list of all the supplements you are taking, including doses and brands, and share it with your GP or other healthcare providers. Avoid stacking multiple high‑dose products with overlapping ingredients, especially minerals, fat‑soluble vitamins, fish oil and herbal blends. If you are correcting a specific deficiency, favour simple, single‑nutrient products and separate other minerals and high‑dose vitamin C to optimise absorption. If you take medications, always seek professional advice before introducing any new supplement or herbal product.

I always advise people to only ever take what they really need and supplements should compliment an already healthy diet, not ever in place of food.