Supplements and Medications: What You Can Combine and What You Can't
You take levothyroxine in the morning, plus an iron supplement — sounds sensible, but it's a problem. Iron blocks the absorption of levothyroxine by up to 50%. "Herbal" or "natural" doesn't mean harmless, and over-the-counter doesn't mean interaction-free.
The golden rule: a time gap solves most problems
Medications first, supplements later with a gap — usually 2–4 hours. If you take a time-critical medication in the morning, move your supplements to the evening.
These four minerals are the most common disruptors. They form complexes with active substances in the gut and prevent their absorption — milk counts too, due to its calcium content.
Extremely sensitive — everything interferes. Take on an empty stomach with water; don't eat or take supplements for 30–60 minutes afterwards.
Vitamin K → weakens blood thinners
⛔ No vitamin K while on warfarin — without medical advice
Vitamin K promotes blood clotting — warfarin inhibits it. A sudden increase in vitamin K (via a supplement or green leafy vegetables) can dangerously weaken anticoagulation and throw INR values 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 available without a prescription
St John's wort activates a liver enzyme (CYP3A4) that breaks down numerous medications faster — they work less effectively or not at all.
Anyone taking prescription medications should NOT take St John's wort without consulting their doctor first.
Omega-3 fatty acids: high-dose + blood thinners = bleeding risk
Over 3 g EPA+DHA per day can affect blood clotting. In combination with aspirin, warfarin, or DOACs, the risk of bleeding increases. Moderate doses (1–2 g/day) are generally unproblematic — still worth discussing with your doctor.
Which Medications "Deplete" Nutrients?
Some medications consume or block nutrients as a side effect. In these cases a supplement may actually be beneficial — but with the right time gap.
With breakfast (at least 60 min after levothyroxine): vitamin D (with a fatty meal) · vitamin B12.
Lunchtime
At least 2h after breakfast: iron supplement with orange juice, without coffee or tea — or antibiotic (2h gap from iron/calcium/magnesium).
Evening
Evening: magnesium (supports sleep, no longer conflicts with morning medications) · simvastatin (works best in the evening).
Frequently Asked Questions
It depends on the medication. For levothyroxine, antibiotics (doxycycline, ciprofloxacin), and bisphosphonates: no — keep a gap of at least 2–4 hours. For blood pressure medications and antidepressants: generally no problem. Taking magnesium in the evening is often the simplest solution.
Not in itself — but in combination with prescription medications it can massively reduce their effectiveness. Particularly critical with antidepressants (serotonin syndrome), the contraceptive pill, and immunosuppressants. Never take it without consulting your doctor first.
Absolutely. Even over-the-counter supplements can cause interactions. Add all your supplements to your medication plan and show it at every doctor's appointment.
Yes — it's actually recommended. Magnesium is needed for the activation of vitamin D in the body. Without sufficient magnesium, vitamin D cannot work properly. The combination is beneficial and unproblematic.
Yes — coffee and black tea contain tannins that inhibit the absorption of iron, calcium, and zinc. Allow 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 instantly whether any conflicts exist.
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. Last updated: March 2026.
