Iron oral (salts: sulfate, bisglycinate, fumarate, gluconate, polymaltose; heme iron polypeptide, lactoferrin)

Mechanism

Iron belongs to the same multivalent cation list in the label. Chelation with fluoroquinolones is among the most pronounced described: the antibiotic's bioavailability falls substantially when the two meet in the stomach.

Symptoms

Cystitis or prostatitis that relapses shortly after the course ends, with the same organism on the follow-up culture.

Management

The same interval applies: 1–2 hours before or 4 hours after. If iron is being taken for anaemia and cannot pause, concentrate it in a single evening dose and spread the antibiotic across the rest of the day. In serious infection the prescriber may prefer a class without this problem.

Mechanism

Iron is named in the label alongside calcium and magnesium as a cation that reduces tetracycline absorption. The binding works both ways: the iron that gets absorbed also drops, so neither the antibiotic nor the anaemia treatment performs as planned.

Symptoms

An antibiotic course without the expected response and, in parallel, ferritin that fails to rise at the 4–8 week check.

Management

At least 2–3 hours apart. For short courses the practical arrangement is iron with dinner and doxycycline in the morning. If the course is long, repeat ferritin at the end to confirm the iron treatment is making progress.

Mechanism

Iron forms a stable, poorly soluble complex with levothyroxine. The AEMPS label lists iron supplements among substances that reduce absorption of the hormone, and NIH ODS names levothyroxine among the drugs iron interacts with. The pairing is common: iron deficiency anaemia and hypothyroidism often occur in the same patient.

Symptoms

TSH rises on an unchanged dose, with fatigue and cold intolerance the patient puts down to the anaemia. The overlap misleads: both conditions cause tiredness.

Management

Levothyroxine in the morning on an empty stomach, iron after a meal or in the evening, 4–5 hours apart. Iron works better with vitamin C and away from dairy. When iron is started in a patient stable on levothyroxine, repeat TSH after 6–8 weeks: the dose may need adjusting for as long as the iron course lasts.

Mechanism

Iron falls into the category of other oral medicines the label requires to be separated from alendronic acid. On top of that, iron's ideal schedule (fasting) collides head-on with the bisphosphonate, which occupies the same slot.

Symptoms

Ferritin that does not recover and flat densitometry: both treatments run at half strength because they compete for the same time slot.

Management

Alendronate keeps the fasting slot and iron moves to mid-morning or evening, with vitamin C and away from dairy. With weekly alendronate the conflict is solved by fixing one day for the bisphosphonate and shifting iron only on that morning.

Mechanism

Calcium interferes with the absorption of both heme and non-heme iron, and it is the only mineral described with that dual effect. They compete for enterocyte transporters, and calcium also acts inside the cell on iron's exit into the blood. Competition peaks on an empty stomach, which is exactly how iron is meant to be taken.

Symptoms

Ferritin rising slowly despite correct iron dosing, with fatigue that persists. In pregnancy and in women with heavy periods the consequence arrives sooner, because the reserve margin is smaller.

Management

Space them by at least two hours: iron in the morning on an empty stomach with vitamin C, calcium with lunch or dinner. Complexes combining iron and calcium in one capsule deliver less than the label indicates. During anaemia treatment, calcium does not go in the same dose as iron.

Mechanism

Non-heme iron needs an acid environment to convert from ferric to ferrous, the form the gut absorbs. NIH ODS states that proton pump inhibitors can potentially reduce iron absorption. The practical consequence: iron deficiency anaemia in someone taking omeprazole responds to oral treatment worse than expected.

Symptoms

Ferritin that rises little or not at all after 8–12 weeks of well-taken oral iron, with no bleeding to explain it.

Management

Take iron with vitamin C, which assists the conversion gastric acid no longer performs. Keep at least 2 hours between iron and the inhibitor. If ferritin is still flat at 12 weeks, reconsider the route: with documented malabsorption, intravenous iron achieves what oral cannot.

Mechanism

Iron and zinc share the DMT1 transporter at the enterocyte brush border. Taken together on an empty stomach at supplement doses, iron, usually present in far larger amounts, displaces zinc. With food in between, the competition dilutes because other ligands come into play.

Symptoms

On the zinc side: altered taste, slow wound healing, more minor respiratory infections. These are rarely attributed to capsule timing.

Management

Two hours apart resolves most of it. If iron treatment is long-term and zinc is also prescribed, place them at opposite ends of the day. With meals both are better tolerated, at the cost of slightly less iron absorption.

Mechanism

This pair does not subtract, it adds. Ascorbic acid reduces ferric iron to ferrous, the form the enterocyte absorbs, and counteracts the phytates and polyphenols that bind it. It is the only combination on this list that is sought deliberately.

Symptoms

No adverse effects of its own. At very high vitamin C doses alongside iron some people notice more stomach upset, which settles on a lower dose.

Management

Use it: iron with a glass of orange juice, or with vitamin C built into the tablet, performs better than iron alone. What does help is keeping it away from tea, coffee and dairy, which pull the other way. In haemochromatosis or iron overload this boost stops being desirable.

Mechanism

Curcumin binds iron in the laboratory and reduces liver and spleen stores in animal models, which raised the fear that it would ruin anaemia treatment. The randomised trial that tested this in healthy adults found no impairment of acute iron absorption with bioavailable curcumin, at any of the ferrous sulphate doses studied. The concern has a biochemical basis, not a clinical one.

Symptoms

No clinical picture from this combination has been described in humans.

Management

There is no need to stop curcumin because of iron. In someone with low ferritin on active anaemia treatment, the sensible step is to check ferritin at 8–12 weeks as in any other case, without attributing a slow response to curcumin in advance when it usually has other causes: poor tolerance, dosing with dairy, or unidentified bleeding.

Mechanism

Both are divalent cations sharing part of the absorption machinery, but the interference described between them is far weaker than calcium with iron. At usual supplement doses the effect is small and without demonstrated clinical consequence.

Symptoms

No symptoms attributable to this pair.

Management

No need to rearrange the regimen for this combination. If iron is being taken for diagnosed anaemia and maximum yield matters, spacing it a couple of hours from magnesium costs nothing and fits with evening magnesium.