Key takeaways
- Iron + vitamin C: together. It is the one pairing with an authorised EU claim: vitamin C increases iron absorption.
- Iron: away from tea, coffee, milk, eggs and calcium by about 2 hours, and at a different time from zinc.
- Medicines first. Magnesium, zinc and iron block some antibiotics, and magnesium blocks osteoporosis tablets; calcium, iron and magnesium block thyroid tablets (levothyroxine) unless you leave 4 hours.
- Vitamin D with a meal that has some fat was absorbed about a third better in one trial. K2 should not be added if you take warfarin-type blood thinners without asking your doctor.
- Zinc above 25 mg a day for weeks can cause copper deficiency. Magnesium is better tolerated split over the day.
The short answer
You don’t need a spreadsheet for most supplements. Three rules cover nearly everything: take iron with vitamin C and away from calcium, dairy, tea and coffee; keep minerals (magnesium, zinc, calcium, iron) away from some antibiotics, osteoporosis tablets and thyroid tablets; and don’t take high-dose zinc for weeks. The tables below give the gaps the official sources use. If you take a prescription medicine, your pharmacist can check your exact list in a minute.
Supplement with supplement, food or drink
| Combination | What happens | What to do |
|---|---|---|
| Iron + vitamin C | Vitamin C increases iron absorption. This is an authorised EU health claim | Take them together, for example iron with a glass of orange juice |
| Iron + tea, coffee, milk, eggs | They reduce how much iron gets into your body | Leave about 2 hours after the iron (NHS) |
| Iron + calcium (also magnesium and antacids) | Calcium reduces the effect of iron | Keep them 2 hours apart (iron medicine leaflets) |
| Iron + zinc | Iron can stop zinc working as well as it should | Take them at different times of day |
| High-dose zinc, for weeks | Zinc blocks copper absorption and can cause copper deficiency | Stay at 25 mg of zinc a day or less from supplements unless a doctor advises more |
| Vitamin D + a meal with some fat | In one trial, vitamin D taken with a meal containing fat was absorbed about 32 % better than with a fat-free meal | Take it with your main meal |
| Calcium carbonate + food | Carbonate is absorbed best with food; citrate is absorbed well with or without | Take carbonate with a meal |
Supplement with medicines
| Combination | What happens | What to do |
|---|---|---|
| Magnesium or zinc + tetracycline or quinolone antibiotics (for example doxycycline, ciprofloxacin) | The mineral binds the antibiotic, which then works less well | Take the antibiotic at least 2 hours before or 4 to 6 hours after the supplement (NIH) |
| Iron + the same antibiotics | Same problem | Keep them at least 2 hours apart (iron medicine leaflets) |
| Magnesium + bisphosphonates (for example alendronate) | Less of the osteoporosis medicine is absorbed | Keep them at least 2 hours apart (NIH) |
| Calcium, iron or magnesium + levothyroxine | Less thyroid hormone is absorbed | Leave at least 4 hours; a multivitamin with these minerals counts too (NHS, Mayo Clinic) |
| Vitamin K2 + warfarin-type blood thinners (vitamin K antagonists) | K2 weakens these medicines, and sudden changes in vitamin K intake are risky | Don’t start or stop K2 without your doctor or anticoagulation clinic |
This is not a complete interaction list. Proton pump inhibitors and some diuretics, for example, can change your magnesium levels over time. Tell your pharmacist every supplement you take, including multivitamins.
When in the day
| Supplement | Best timing | Why |
|---|---|---|
| Iron | On an empty stomach (30 minutes before or 2 hours after eating) if you tolerate it; with food if it upsets your stomach | NHS. In iron-depleted women, a single morning dose on alternate days was absorbed better than daily split doses |
| Vitamin D | With a meal that contains some fat | Better absorption |
| Magnesium | Split over the day, at least two doses | The BfR advises this for tolerance |
| Melatonin | Shortly before bed | The EU claim (falling asleep faster) applies to 1 mg taken close to bedtime |
| Creatine | Any time, every day | The EU claim is for 3 g a day; it doesn’t depend on timing |
| Calcium | Carbonate with a meal, ideally 500 mg or less at a time | The NIH notes absorption is best at doses of 500 mg or less |
A practical day
If you take several supplements and a medicine, a simple split solves most of the table:
- On waking: levothyroxine (if prescribed), with water only.
- Mid-morning, 2 hours away from coffee: iron with a glass of orange juice, if you take iron.
- Lunch or dinner: vitamin D, omega-3 and a multivitamin with food.
- Evening: magnesium (or half your dose), zinc if you take it on its own.
Check the totals too. A multivitamin plus a separate zinc, magnesium or vitamin D product can push you over the safe upper levels. Our maximum levels by country show the limits, and how to read a supplement label shows where to find the real amount per daily dose. For iron specifically, read iron and tiredness first: iron only helps if you are low in it.
FAQ
Which supplements should not be taken together?
The main ones: iron with calcium, tea, coffee or dairy (leave about 2 hours), iron with zinc (take them at different times), and high-dose zinc for a long time, which lowers copper. With medicines: magnesium, zinc and iron with tetracycline or quinolone antibiotics, magnesium with bisphosphonates, and calcium, iron or magnesium with levothyroxine.
Can I take iron and vitamin C together?
Yes, and you should. "Vitamin C increases iron absorption" is an authorised EU health claim. A glass of orange juice with your iron tablet or with a plant-based meal does the job.
Can I take vitamin D and magnesium together?
Yes. There is no known problem taking them at the same time. Take vitamin D with a meal that contains some fat, and keep magnesium from supplements at or below 250 mg a day.
Can I take zinc and magnesium together?
At normal doses, yes. The interaction to watch with zinc is copper: more than 25 mg of zinc a day for weeks can lower copper. And keep zinc and iron at different times of day.
How long should I wait between levothyroxine and supplements?
UK and US guidance: at least 4 hours between levothyroxine and calcium, iron or magnesium, including multivitamins that contain them. Most people take levothyroxine on waking and supplements with lunch or dinner. Ask your pharmacist about anything else you take.
Is it better to take supplements in the morning or at night?
For most supplements the time of day matters less than taking them regularly. The exceptions: melatonin shortly before bed (that is the condition of its EU claim), iron on an empty stomach if you tolerate it, and vitamin D with a meal.
Sources
- Regulation (EU) No 432/2012 — list of permitted health claims (vitamin C and iron absorption; melatonin; creatine)
- NHS — About ferrous sulfate (empty stomach; 2-hour gap before tea, coffee, dairy, eggs)
- Ferrous sulfate 200 mg tablets — patient leaflet (calcium and magnesium supplements, antibiotics, 2 hours)
- HSE (Ireland) — Ferrous fumarate (calcium reduces its effect; iron reduces zinc)
- NHS — Others: zinc (no more than 25 mg a day from supplements)
- EFSA — Summary of tolerable upper intake levels (2024)
- Dawson-Hughes B et al. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet, 2015
- Verbraucherzentrale — Vitamin K products: what makes sense? (BfR on K2 and vitamin K antagonists; German)
- NIH Office of Dietary Supplements — Magnesium (interactions with medications)
- NIH Office of Dietary Supplements — Zinc (interactions with medications)
- NIH Office of Dietary Supplements — Calcium (carbonate with food, citrate with or without)
- NHS — Taking levothyroxine with other medicines and supplements
- Mayo Clinic — Hypothyroidism: can calcium supplements interfere with treatment?
- BfR — Daily maximum for magnesium in food supplements (split into at least two doses; German)
- Stoffel NU et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women. Lancet Haematol, 2017