Oral and intravenous iron therapy

01Purpose and principlesWhat the treatment does and how it fits into care.

Oral iron is effective, inexpensive and suitable for many stable patients when intestinal absorption and adherence are adequate. Ferrous sulfate 200 mg, ferrous fumarate 210 mg and ferrous gluconate 300 mg tablets contain different amounts of elemental iron, but BSG recommends beginning with one tablet daily rather than older multiple-dose regimens. Alternate-day dosing may improve fractional absorption and tolerability for some people. Nausea, dyspepsia, constipation, diarrhoea and dark stools are common and should be anticipated with practical advice rather than interpreted automatically as allergy.

Intravenous iron delivers a larger amount without gastrointestinal absorption. It is useful in active inflammatory bowel disease, significant malabsorption, chronic kidney disease pathways, persistent intolerance, ongoing loss exceeding oral delivery and situations where a faster response is clinically important. The diagnosis and source investigation still continue. Ferric carboxymaltose and ferric derisomaltose permit larger single infusions than iron sucrose, but their dosing formulas and maximums differ. The selected product, weight and haemoglobin determine the prescription and infusion record.

Safety requires more than observing for immediate allergy. Extravasated iron can cause long-lasting skin staining. Ferric carboxymaltose can increase fibroblast growth factor signalling, renal phosphate wasting and, after repeated exposure, muscle pain, weakness, fractures or osteomalacia. Infection and inflammatory context should be reviewed before parenteral treatment. Response is assessed after enough time for erythropoiesis and redistribution; measuring ferritin immediately after infusion can falsely imply stable repletion.

Key points

Confirm iron deficiency or a guideline-supported functional iron indication before treatment. Low haemoglobin alone does not justify iron when haemolysis, B12 deficiency, renal disease or marrow pathology may dominate.
For most stable adults, BSG prefers one tablet daily of ferrous sulfate, fumarate or gluconate; if intolerance occurs, every-other-day dosing is a supported alternative before abandoning oral therapy.
The iron salt and elemental iron content are not interchangeable. Prescribe the exact product and tablet strength and check the BNF rather than writing 'iron tablets'.
Absorption is reduced by food and several medicines, but fasting administration can worsen intolerance. Agree a workable regimen and separate interacting treatments using current BNF advice.
Choose intravenous iron when oral treatment is ineffective, not tolerated, not absorbed, contraindicated or too slow for the clinical need; calculate total requirement using that product's SmPC.
Intravenous formulations differ in maximum single dose, dilution, infusion rate and repeat interval. Never transfer instructions between ferric carboxymaltose, ferric derisomaltose and iron sucrose.
All intravenous products can cause hypersensitivity and extravasation. Give them where trained staff can monitor the patient and manage anaphylaxis, and observe for at least 30 minutes after each administration.
Repeated or high-dose ferric carboxymaltose can cause clinically important hypophosphataemia and osteomalacia; identify risk, monitor phosphate where advised and reconsider the formulation if persistent.

02Indications, selection and cautionsWho may benefit, who needs urgent treatment and important alternatives.

Suitable for oral therapy

Stable confirmed deficiency, intact absorption, no urgent replacement need and willingness to follow a tolerable regimen supports oral treatment.

Oral intolerance or failure

Persistent gastrointestinal adverse effects, poor adherence, malabsorption, continued loss or absent haemoglobin response despite correct use justifies route and diagnosis review.

Intravenous indication

Clinically important deficiency with ineffective, intolerable or unlikely oral absorption, or a need for faster repletion, supports product-specific intravenous treatment.

Infusion hypersensitivityRed flag

Flushing, urticaria, wheeze, airway swelling, hypotension or collapse during infusion requires immediate cessation, ABCDE assessment and the anaphylaxis pathway.

Phosphate complication

New fatigue, proximal weakness, bone pain or fracture after repeated ferric carboxymaltose raises renal phosphate wasting and osteomalacia risk.

03Assessment before treatmentTests and checks that guide safe selection.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
FBC, ferritin, transferrin saturation and CRPFirst step
Why
Confirm indication, severity and inflammatory modification before treatment.
Interpretation and limitations
Low ferritin supports absolute deficiency; inflammation may preserve ferritin while saturation is low. Do not infer iron deficiency from anaemia alone or measure immediately after an infusion as steady state.
02
Weight and product-specific dose calculation
Why
Calculate the total iron deficit and safe single dose for intravenous treatment.
Interpretation and limitations
Use current measured weight, haemoglobin and the selected SmPC's table or formula. Independent checking should confirm product, total dose, maximum, dilution, rate and repeat interval.
03
Renal, liver and infection assessment
Why
Identify comorbidity that changes route, monitoring or benefit-risk.
Interpretation and limitations
CKD may follow a specialist functional-iron pathway; uncontrolled infection and significant liver disease require individual review rather than automatic infusion.
04
Serum phosphate when risk is present
Why
Detect ferric-carboxymaltose-related phosphate wasting and prevent bone complications.
Interpretation and limitations
Check before repeated high-dose or long-term exposure in at-risk patients and recheck when symptoms occur. Persistent low phosphate should prompt treatment review and alternative formulation consideration.
05
Early FBC response
Why
Confirm that delivered iron is producing effective erythropoiesis.
Interpretation and limitations
A rising haemoglobin supports response. Failure prompts review of adherence, dose, absorption, ongoing bleeding, inflammation, mixed deficiency and the original diagnosis rather than automatic repeat infusion.

04Treatment approachPreparation, options, escalation and aftercare.

01OralStart a tolerable daily regimenFirst stepConfirmed iron deficiency is stable and oral absorption is expected to be adequate.

1PreferredPreferred treatment is one daily tablet of ferrous sulfate, fumarate or gluconate with the exact product documented, plus counselling on adverse effects, food and medicine interactions and child-safe storage.
2AlternativeEscalationIf intolerance occurs, the supported alternative is one tablet every other day, another salt or a carefully agreed food schedule rather than unstructured dose escalation.
3EscalationCheck an early haemoglobin response and continue after normalisation to replenish stores; escalate absent response to a cause and route review.

02IntravenousCalculate and administer one formulationOral iron is ineffective, intolerable, malabsorbed or too slow for the supported clinical need.

1Confirm indication and source plan, select the licensed formulation, calculate total requirement and single-dose limit from its SmPC, and complete an independent prescription and administration check.
2Administer through the formulation-specific dilution and rate in a monitored area with resuscitation capability, checking the cannula and asking about pain throughout to prevent extravasation.
3Observe during administration and for at least 30 minutes afterwards, document product and cumulative dose, and arrange response and phosphate monitoring according to formulation and risk.

03Failure or reactionReassess before giving more ironHaemoglobin does not respond, intolerance continues, phosphate falls or an infusion reaction occurs.

1Stop an infusion immediately for possible serious hypersensitivity, use ABCDE and the local anaphylaxis protocol, and document the exact reaction and product rather than a generic 'iron allergy'.
2For non-response, reassess ongoing bleeding, malabsorption, inflammation, renal disease, mixed haematinic deficiency, marrow disease and whether enough iron was actually delivered.
3AlternativeEscalationUse an alternative oral schedule, different intravenous formulation or specialist therapy only after the mechanism is clear; escalate persistent hypophosphataemia, bone pain or fracture for metabolic review.

05Regimens, contraindications and interactionsTreatment details and the circumstances that modify them.

Supplies approximately 65 mg elemental iron per tablet for stable oral repletion.

Ferrous sulfate

One 200 mg tablet by mouth once daily is a common adult BSG starting regimen; reduce to every other day if gastrointestinal intolerance limits adherence.

Common effects are nausea, abdominal discomfort, constipation, diarrhoea and dark stools. Check BNF spacing from interacting medicines, avoid overdose in children and investigate persistent non-response or bleeding.

Supplies approximately 68 mg elemental iron per tablet when another ferrous salt is preferred.

Ferrous fumarate

One 210 mg tablet by mouth once daily is a BSG-listed alternative, with every-other-day administration if needed for tolerance.

Gastrointestinal effects and interactions are similar to other oral salts. Verify tablet strength because formulations differ and do not prescribe by salt name without dose.

Provides approximately 35 mg elemental iron per tablet and may suit a patient who tolerates a lower elemental amount better.

Ferrous gluconate

One 300 mg tablet by mouth once daily is a lower-elemental-iron BSG-listed alternative for selected adults.

Lower elemental content does not guarantee tolerance or response. Confirm the marketed strength, interaction spacing, early haemoglobin response and duration needed to replenish stores.

Provides high-dose intravenous replacement when oral therapy is unsuitable or a faster correction is clinically needed.

Ferric carboxymaltose

Use the Ferinject SmPC calculation; a single dose must not exceed 20 mg iron/kg or 1,000 mg iron, and the maximum weekly dose is 1,000 mg.

Monitor for hypersensitivity and extravasation. Repeated high-dose exposure can cause symptomatic hypophosphataemia and osteomalacia; check phosphate in at-risk patients and with compatible symptoms.

Allows high-dose intravenous replacement in selected adults using a formulation-specific calculation and rate.

Ferric derisomaltose

Use the Monofer SmPC calculation; up to 20 mg iron/kg may be given as one infusion, while a larger requirement is divided by the licensed interval.

Hypersensitivity and extravasation remain possible. Do not copy ferric-carboxymaltose dose limits or infusion instructions; check weight, total need, infection and the current SmPC.

06Complications, monitoring and follow-upAdverse effects, response and longer-term review.

For oral treatment, review gastrointestinal tolerance, adherence, interacting medicines and early haemoglobin response before deciding that the preparation has failed.
Continue oral iron after haemoglobin normalises for the cause-appropriate replenishment period and monitor recurrence when ongoing loss cannot be eliminated.
During intravenous treatment, monitor observations and symptoms under the product protocol, inspect the cannula, stop immediately for hypersensitivity or extravasation and observe for at least 30 minutes after administration.
Document formulation, batch where required, calculated deficit, delivered elemental amount and remaining requirement so different products are not inadvertently combined incorrectly.
Check phosphate with repeated high-dose ferric carboxymaltose, long-term treatment, risk factors or bone and muscle symptoms, and act on persistent abnormality.
Delay post-infusion ferritin interpretation until redistribution is clinically meaningful and use haemoglobin and symptoms to assess early effectiveness.

07Special situationsVariants, exceptions and circumstances that change the usual approach.

Elemental iron is the active comparison

Salt weight and elemental content differ. Ferrous sulfate 200 mg and ferrous fumarate 210 mg provide similar elemental amounts, while gluconate provides less.

More tablets can mean less treatment

Frequent dosing increases gastrointestinal burden and hepcidin-mediated absorption limits. A tolerable daily or alternate-day plan can deliver better real-world adherence.

Formulations are not interchangeable

Intravenous products have distinct iron complexes, maximum doses, infusion rates and adverse-effect profiles. Prescribing must name and follow one SmPC.

Phosphate symptoms mimic anaemia

Fatigue and weakness after ferric carboxymaltose may be attributed to persistent anaemia. Bone pain or repeated exposure should prompt phosphate assessment.

Infusion does not find the source

Rapid iron delivery can normalise haemoglobin while occult bleeding continues. Source investigation and recurrence monitoring remain separate responsibilities.

08Common pitfallsFrequent interpretation and management errors.

01
Prescribing 'iron tablets' without the salt, strength and schedule.
02
Using older multiple-daily dosing automatically despite intolerance and current BSG guidance.
03
Calling dark stools harmless without assessing new melaena symptoms.
04
Copying one intravenous formulation's maximum dose and rate to another.
05
Giving repeated ferric carboxymaltose without phosphate risk assessment.
06
Interpreting ferritin immediately after infusion as durable store repletion.
07
Repeating iron for non-response without reconsidering bleeding, malabsorption, inflammation and diagnosis.