Key points
Real conflicts are few. The most important ones involve medicines — thyroid pills and some antibiotics — and are solved by spacing doses apart (Mana DL Medicina (B Aires) 2025. PMID 41109285; Ogawa R Drugs 2011. PMID 21942976).
Calcium with iron is no disaster. In short experiments, calcium lowered iron absorption; in long-term studies haemoglobin did not fall, but the more calcium, the lower the ferritin (Minihane AM Am J Clin Nutr 1998. PMID 9665102; Abioye AI J Nutr 2021. PMID 33758936).
Vitamin C with iron is optional. In 440 adults with iron deficiency anaemia, iron without vitamin C raised haemoglobin comparably to iron plus vitamin C (Li N JAMA Netw Open 2020. PMID 33136134).
Fat improves vitamin D absorption. In 50 older people, after a single dose, the peak blood level of vitamin D3 was 32% higher with a breakfast that contained fat than with a fat-free one (Dawson-Hughes B J Acad Nutr Diet 2015. PMID 25441954).
Warfarin: stability, not a ban. Restricting vitamin K in food apparently does not improve treatment; a stable diet may matter more (Violi F Medicine (Baltimore) 2016. PMID 26962786).
What really matters
Thyroid pills. Levothyroxine is absorbed less well together with calcium and iron salts; a review advises spacing them 3–4 hours apart (Mana DL Medicina (B Aires) 2025. PMID 41109285). Magnesium interferes less: in 15 healthy volunteers, magnesium aspartate lowered thyroxine absorption by 12% (significant), magnesium citrate by 7% (not significant) (Attinger MC Clin Transl Sci 2025. PMID 41221788). More in the article Iron and thyroid pills.
Two groups of antibiotics. Tetracyclines (for example, doxycycline) and fluoroquinolones (for example, ciprofloxacin) bind to metal ions from antacids (chelation) and are absorbed less well. A review advises taking the antacid 4 hours before the antibiotic or 2 hours after (Ogawa R Drugs 2011. PMID 21942976). For supplements the interval has not been studied — it is carried over from antacids; the exact timing differs for each antibiotic, so check the package leaflet.
High-dose zinc and copper. The handbook advises monitoring copper as soon as you take more than 25 mg of zinc a day for longer than 6 weeks (Burgerstein, Handbuch Nährstoffe 2015, p. 112, 136). A systematic review collected 37 cases of copper deficiency caused by zinc at doses from roughly 50 to more than 1500 mg a day; the review has no data for doses of 25–50 mg. Almost all of them had anaemia; after zinc was stopped and copper given, the blood usually recovered, while nerve problems recovered more slowly and not always fully (Dutta A Biol Trace Elem Res 2026. PMID 42087025). On reasonable doses — in the article Zinc: why you need it and how much is safe.
Warfarin and vitamin K. A systematic review (2 interventional and 9 observational studies) found conflicting data: an effect may be noticeable only at intakes above 150 µg of vitamin K a day, and restricting vitamin K in food apparently does not improve treatment. The authors advise keeping vitamin K intake roughly constant (Violi F Medicine (Baltimore) 2016. PMID 26962786).
What matters less than it seems
Calcium and iron. In 14 volunteers, 400 mg of calcium with each meal lowered iron absorption over a day from 15.8% to 4.7%. But in 11 adults with normal iron stores, half a year of taking 1200 mg of calcium a day did not change haemoglobin or ferritin (iron stores) (Minihane AM Am J Clin Nutr 1998. PMID 9665102). In a meta-analysis, calcium in short experiments lowered iron absorption by an average of 5.6 percentage points; haemoglobin did not fall, but the more calcium, the lower the ferritin (Abioye AI J Nutr 2021. PMID 33758936).
Vitamin C and iron. The handbook states that vitamin C markedly increases iron uptake (Burgerstein, Handbuch Nährstoffe 2015, p. 89, 114–115). But in a randomised trial, 440 adults with iron deficiency anaemia (97% women) took one 100 mg ferrous succinate tablet (this is the weight of the salt; it contains less iron) three times a day — with 200 mg of vitamin C or without. Over 2 weeks haemoglobin rose by 20.0 g/L (2.00 g/dL) and 18.4 g/L (1.84 g/dL): the difference fell within the preset margin of 10 g/L (1 g/dL) (Li N JAMA Netw Open 2020. PMID 33136134).
Coffee and tea with iron. Tannins in tea and coffee reduce iron uptake (Burgerstein, Handbuch Nährstoffe 2015, p. 113). A cup of coffee lowered iron absorption from a hamburger meal by 39%, and tea by 64%. Coffee an hour before the meal did not interfere, while coffee an hour after the meal interfered as much as coffee drunk with it (Morck TA Am J Clin Nutr 1983. PMID 6402915). For tea and iron tablets, timing has not been studied.
Fat-soluble vitamins and food. With a single dose of 50,000 international units (IU; 1,250 µg) of vitamin D3, a breakfast with fat gave a 32% higher peak blood level than a fat-free one (Dawson-Hughes B J Acad Nutr Diet 2015. PMID 25441954); vitamin D levels months later were not checked. More in the article Vitamin D in winter.
What to space apart, and by how many hours
- Levothyroxine and calcium, iron — 3–4 hours (Mana DL Medicina (B Aires) 2025. PMID 41109285); magnesium — separately, the interval has not been studied (Attinger MC Clin Transl Sci 2025. PMID 41221788).
- Tetracyclines, fluoroquinolones and antacids — the antacid 4 hours before or 2 hours after the antibiotic (Ogawa R Drugs 2011. PMID 21942976). For supplements the interval has not been studied — it is carried over from antacids; the exact timing differs for each antibiotic, so check the package leaflet.
- Iron and coffee — coffee an hour before a meal (Morck TA Am J Clin Nutr 1983. PMID 6402915).
- Calcium and iron — with normal iron stores, spacing them apart is, according to the available data, not necessary; data are still scarce (Abioye AI J Nutr 2021. PMID 33758936).
What the studies showed
| What was tested | Who took part | Result |
|---|---|---|
| Calcium and iron, 1998 | 14 volunteers; 24 people for half a year | Absorption over a day 15.8 → 4.7%; after half a year haemoglobin and ferritin did not change |
| Calcium and iron, meta-analysis, 2021 | Randomised and crossover studies | Absorption 5.6 percentage points lower; haemoglobin did not fall, but the more calcium, the lower the ferritin |
| Vitamin C with iron, 2020 | 440 adults with iron deficiency anaemia | Rise in haemoglobin comparable: difference within 10 g/L (1 g/dL) |
| Coffee and tea with a meal, 1983 | People with normal iron levels | Iron absorption −39% and −64% |
| Levothyroxine and magnesium, 2025 | 15 healthy volunteers | Thyroxine absorption −12% (significant) and −7% (not significant) |
| High-dose zinc, 2026 | 37 reported cases | Copper low in all |
| Vitamin K and warfarin, 2016 | 11 studies | Data conflicting |
| Vitamin D3 with fat, 2015 | 50 older people | Peak blood level 32% higher |
Not everything is equally proven: that taking calcium and iron separately improves treatment results has not been shown (Abioye AI J Nutr 2021. PMID 33758936). The handbook advises taking calcium and manganese on different days (Burgerstein, Handbuch Nährstoffe 2015, p. 117–118): in 6 adults, calcium almost completely blocked the entry of manganese into the blood (Freeland-Graves JH J Am Coll Nutr 1991. PMID 2010579), but the benefit of taking them on different days has not been studied. The handbook’s warning that high doses of vitamin C weaken anticoagulants (Burgerstein, Handbuch Nährstoffe 2015, p. 91) was not checked in this set of sources.
Who should be more careful
- People taking levothyroxine: if you start or stop calcium, iron or magnesium, tell your endocrinologist.
- People being treated with tetracyclines or fluoroquinolones: check the interval with minerals with your doctor.
- People taking more than 25 mg of zinc a day for longer than 6 weeks: discuss copper monitoring with your doctor; the review has no data for doses of 25–50 mg.
- People taking warfarin: do not start or stop vitamin K supplements without your doctor.
Bottom line
Most vitamins and minerals can be taken together. A few pairs matter: levothyroxine and two groups of antibiotics with minerals, high doses of zinc with copper, warfarin with sudden changes in vitamin K. You can put together a regimen that suits your medicines at a consultation.
This material is for information only and does not replace a doctor’s consultation. Do not change how you take medicines and supplements without agreeing it with your doctor.
