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GuideMarch 2026· 7 min read
Dietary supplements and medications: what you may combine and what you may not
In the morning you take levothyroxine, along with an iron supplement — it sounds sensible, but it is a problem. Iron blocks the absorption of levothyroxine by up to 50 %. "Herbal" or "natural" does not mean harmless, and over-the-counter does not mean free of interactions.
The golden rule: spacing out the timing solves most problems
Medications first, supplements with a gap afterwards — usually 2–4 hours. If you take a time-critical medication in the morning, move supplements to the evening.
These four minerals are the most common disruptors. In the gut they form complexes with active ingredients and prevent their absorption — milk counts too, because of its calcium.
Extremely sensitive — everything blocks them. On an empty stomach with water, then eat or supplement nothing for 30–60 min.
Vitamin K → weakens blood thinners
⛔ No vitamin K while on Marcumar — without consulting
Vitamin K promotes blood clotting — Marcumar/phenprocoumon inhibits it. A sudden lot of vitamin K (through a supplement or green leafy vegetables) can dangerously weaken the blood thinning and send the INR value out of control.
Important: with the newer blood thinners (DOACs such as rivaroxaban, apixaban), vitamin K plays NO role — they work via a different mechanism.
St. John's wort → the underestimated disruptor
⛔ St. John's wort — dangerous despite being over-the-counter
St. John's wort activates a liver enzyme (CYP3A4) that breaks down numerous medications faster — they work more weakly or not at all.
Anyone taking prescription medications should NOT take St. John's wort without consulting a doctor.
Omega-3 fatty acids: high doses + blood thinners = bleeding risk
More than 3 g of EPA+DHA daily can affect blood clotting. In combination with ASA (aspirin), Marcumar or DOACs, the bleeding risk rises. Moderate doses (1–2 g/day) are usually unproblematic — still, discuss it with your doctor.
Which medications "rob" nutrients?
Some medications use up or block nutrients as a side effect. In these cases a supplement can even make sense — but with the right gap.
With breakfast (at least 60 min after levothyroxine): vitamin D (with a fat-containing meal) · vitamin B12.
Midday
At least 2h after breakfast: iron supplement with orange juice, without coffee/tea — or an antibiotic (2h gap from iron/calcium/magnesium).
Evening
In the evening: magnesium (supports sleep, no more conflict with morning pills) · simvastatin (works best in the evening).
Common questions
It depends on the medication. With levothyroxine, antibiotics (doxycycline, ciprofloxacin) and bisphosphonates: no — at least 2–4 hours' gap. With blood-pressure-lowering drugs and antidepressants: usually not a problem. Magnesium in the evening is often the simplest solution.
Not in itself — but in combination with prescription medications it can massively weaken their effect. Especially critical with antidepressants (serotonin syndrome), the pill and immunosuppressants. Never take it without consulting a doctor.
Absolutely. Over-the-counter supplements too can cause interactions. Enter all supplements in your medication plan and show it at every doctor's visit.
Yes — even advisable. Magnesium is needed for the activation of vitamin D in the body. Without enough magnesium, vitamin D cannot work properly. The combination makes sense and is unproblematic.
Yes — coffee and black tea contain tannins that inhibit the absorption of iron, calcium and zinc. Keep at least 30–60 minutes between coffee and these supplements.
Check supplements and medications together
Enter all your medications AND supplements into the brite interaction check — including over-the-counter ones. See immediately whether there are any conflicts.
Medical disclaimer: This page does not replace medical advice. Supplements can affect medications — discuss any supplementation with your doctor or pharmacist, especially if you take prescription medications. As of: March 2026.
