Take the iron tablet with a large glass of water, one hour before or three hours after meals, and keep it at least two hours away from coffee, tea, red wine, dairy, eggs, pulses and bran. Swallow it whole, without chewing. That, in short, is the difference between a treatment that works and one that half works.
Iron is one of the worst-taken supplements there is. Not because people forget, but because almost nobody knows that food and other medicines can block a good part of the absorption, and because the treatment gets abandoned as soon as you feel better — which is exactly before it has achieved anything.
Why the timing matters so much
Iron in food comes in two forms. Haem iron, present in meat, fish and shellfish, is absorbed easily. Non-haem iron, from pulses, nuts, vegetables and fortified cereals, is absorbed far less well and is extremely sensitive to whatever accompanies it. Supplements provide non-haem iron, so they carry that same fragility.
Hence why the AEMPS data sheet for oral iron salts is so explicit about the timing gaps. This is not a courtesy recommendation: these are substances that form complexes with iron in the gut and carry it away unabsorbed.
| What to separate from iron | By how much |
|---|---|
| Tea, coffee, red wine | Avoid large amounts; separate the dose |
| Dairy, cheese, eggs | At least 2 hours |
| Bran, pulses, oilseed cereals | At least 2 hours |
| Calcium or zinc supplements | At least 2 hours |
| Antacids (aluminium, calcium, magnesium salts) | At least 2 hours |
| Thyroxine (thyroid) | At least 2 hours |
| Levodopa, carbidopa, methyldopa, entacapone | At least 2 hours |
| Bisphosphonates (osteoporosis) | At least 2 hours |
| Cyclines, fluoroquinolones, cefdinir (antibiotics) | At least 2 hours |
| Colestyramine | 1-2 h before or 4-6 h after |
Vitamin C goes the other way: it increases iron absorption, both from the supplement and from the non-haem iron in food. That is why it makes sense to take the tablet with an orange or kiwi juice rather than with coffee. This isn’t an internet trick; it appears in the medicine’s own data sheet and in the US National Institutes of Health review.
Proton pump inhibitors — omeprazole, lansoprazole and company — reduce stomach acid and with it the amount of non-haem iron the body manages to extract from food. If you take one long term, that is a fact worth putting on the table.
Not all “mg” of iron are the same iron
On the shelf you’ll see ferrous sulfate, ferrous fumarate, ferrous gluconate and several other forms. The practical difference is how much elemental iron each salt provides per unit of weight:
- Ferrous fumarate: 33% elemental iron
- Ferrous sulfate: 20%
- Ferrous gluconate: 12%
In other words, 300 mg of ferrous gluconate is not equivalent to 300 mg of sulfate. What you need to look at on the pack is the elemental iron figure, which is the one declared.
As a reference for how much iron a healthy person needs per day, these are the NIH recommended intakes:
| Age | Men | Women | Pregnancy | Breastfeeding |
|---|---|---|---|---|
| 1-3 years | 7 mg | 7 mg | ||
| 4-8 years | 10 mg | 10 mg | ||
| 9-13 years | 8 mg | 8 mg | ||
| 14-18 years | 11 mg | 15 mg | 27 mg | 10 mg |
| 19-50 years | 8 mg | 18 mg | 27 mg | 9 mg |
| 51+ years | 8 mg | 8 mg |
Anyone following a vegetarian diet needs around 1.8 times those figures, precisely because their iron is all non-haem.
The tolerable upper limit for adults is 45 mg a day. Iron-only supplements usually exceed it comfortably — many provide 65 mg — because they are designed to correct a diagnosed deficiency, not to be taken indefinitely just in case.
Supplementing without a diagnosis is not harmless
This is the point most often skipped. An iron supplement is not indicated for “tiredness” on its own. The data sheet says it plainly: when treating iron deficiency anaemia, the cause of that iron shortage must also be investigated so it can be treated. In an adult man or a postmenopausal woman, iron deficiency with no obvious explanation is a finding that deserves investigation, not a bottle of tablets.
Iron salts are also contraindicated if you already have more iron than you need, or if your anaemia is not caused by iron deficiency — thalassaemia, refractory anaemia, anaemia from bone marrow failure. And if the deficiency is associated with an inflammatory disorder, oral treatment simply will not be effective.
The way to know which situation you are in is a blood test. The WHO sets the serum ferritin threshold defining iron deficiency at under 15 µg/L in adults and adolescents, and under 12 µg/L in children under five. If infection or inflammation is involved, that threshold rises to 70 µg/L, because ferritin is artificially elevated. That is why the test is interpreted by a doctor and not by a search engine.
The unpleasant effects and the ones that only look alarming
Digestive complaints are common: constipation, diarrhoea, abdominal bloating, abdominal pain and nausea affect up to 1 in 10 people.
Black stools are also common. This effect is harmless and does not mean there is bleeding. It’s worth knowing in advance so you don’t panic — and so you don’t confuse it with a real gastrointestinal bleed, which looks different and usually comes with other symptoms.
Two precautions people overlook:
- Don’t suck, chew or hold the tablet in your mouth. It can cause mouth ulcers and tooth discolouration. Swallow it whole with water.
- If you have difficulty swallowing, ask before starting. An iron tablet lodged in the oesophagus or entering the airway can cause serious injury.
The treatment lasts longer than you think
Correcting the anaemia and refilling the stores are two different things. Haemoglobin recovers before ferritin does, so feeling well does not mean you have finished. The duration depends on how severe the deficiency was, but generally at least three months are needed, and often more, guided by the blood parameters. Don’t stop the treatment early even if you feel better: relapse is the norm.
When to seek advice
Ask for medical assessment if:
- You have tiredness, pallor or breathlessness and have not had a blood test. Don’t start supplementing blind.
- You are an adult man or a postmenopausal woman and iron deficiency has been found with no obvious cause.
- You see blood in your stools, black sticky stools with a strong smell, or you vomit something resembling coffee grounds.
- You have very heavy periods that force you to change protection every one or two hours.
- You are losing weight without trying, or have persistent abdominal pain.
- You have been on treatment for weeks and notice no improvement at all, or the blood results don’t move.
- A persistent cough, coughing blood or difficulty breathing appears after choking on a tablet, even some time later.
- A child has swallowed iron tablets by accident: this is an emergency. Call the toxicology information service (91 562 04 20 in Spain) or go to A&E.
If you’re not sure which supplement fits your situation, or whether it’s safe alongside your other medicines, the pharmacists at Farmàcia Arqué in Salou can go through it with you, no appointment needed — see how to find us. Outside our regular hours, Salou’s duty pharmacy is still open.
This article is general information and does not replace assessment by a healthcare professional. Sources: Ferrous sulfate patient leaflet, CIMA-AEMPS · Iron — Fact Sheet for Health Professionals, NIH Office of Dietary Supplements · WHO guideline on the use of ferritin concentrations to assess iron status