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# Antianaemia agents (iron, folic acid, vitamin B12)

Iron for microcytic iron-deficiency anaemia; folic acid and vitamin B12 for megaloblastic anaemia. Each replaces what the marrow lacks.

Updated 2026-11-01

## What they are

Iron (oral salts such as ferrous sulfate, or intravenous iron such as iron sucrose) restores haemoglobin synthesis. Folic acid and vitamin B12 (cyanocobalamin, hydroxocobalamin) restore DNA synthesis in dividing cells; B12 also protects myelin. The cause of the deficiency is always looked for.

## Mechanism of action

Iron is the core of haem. Folate carries one-carbon units for thymidine and purine synthesis, and B12 is needed to regenerate tetrahydrofolate from methyltetrahydrofolate (methionine synthase) and for methylmalonyl-CoA mutase. Lack of folate or B12 produces large, immature red cells (megaloblasts).

Target: Haem synthesis and DNA synthesis in red cell precursors

## Key points

- Microcytic anaemia with low ferritin = iron deficiency; macrocytic with hypersegmented neutrophils = folate or B12 deficiency
- Folic acid alone in B12 deficiency can mask the anaemia while neurological damage progresses
- Pernicious anaemia needs parenteral B12 for life: oral absorption depends on intrinsic factor
- Folic acid before conception and in early pregnancy prevents neural tube defects
- Iron overdose is a leading cause of fatal poisoning in young children; the antidote is deferoxamine

## Drugs in this class

- [**Ferrous sulfate** The standard oral iron salt for treating and preventing iron-deficiency anaemia. Gut upset is common; overdose is… Transferrin, the plasma iron carrier (iron is built into haemoglobin)](https://durgical.com/en/pharmacology/ferrous-sulfate/)
- [**Iron sucrose** Intravenous iron used mainly in chronic kidney disease, often with an ESA. Patients are watched for allergic reactions… Transferrin, the plasma iron carrier (iron is built into haemoglobin)](https://durgical.com/en/pharmacology/iron-sucrose/)
- [**Folic acid** The synthetic form of folate. It treats folate-deficiency megaloblastic anaemia and, taken before and early in… Dihydrofolate reductase (activates folic acid)](https://durgical.com/en/pharmacology/folic-acid/)
- [**Cyanocobalamin** The common form of vitamin B12 used to treat and prevent B12 deficiency, including pernicious anaemia, by injection… Gastric intrinsic factor (absorption) and the B12-dependent enzymes](https://durgical.com/en/pharmacology/cyanocobalamin/)
- [**Hydroxocobalamin** A form of vitamin B12 for B12 deficiency that stays longer in the body, and the antidote that binds cyanide to form… Methionine synthase (B12 coenzyme); free cyanide in poisoning](https://durgical.com/en/pharmacology/hydroxocobalamin/)

## Others in the class

- ferrous fumarate
- ferrous gluconate
- ferric carboxymaltose
- iron dextran
- ferrous salt + folic acid (pregnancy)

## Test yourself

Think of the answer, then tap the question to see it.

## Which vitamin deficiency causes neurological damage as well as megaloblastic anaemia?

Vitamin B12.

## Why is folic acid alone dangerous in pernicious anaemia?

It improves the blood count but not the nerve damage, which keeps progressing.

## What laboratory sign shows the first response to treatment?

A rise in reticulocytes within days.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 45 (McGraw Hill, 2023); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 33 (McGraw Hill, 2025); [DailyMed - HYDROXOCOBALAMIN injection, solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a3087a39-f218-4f16-a1e7-17c39bb2eb69) (DailyMed (U.S. National Library of Medicine), 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/) (World Health Organization, 2025).

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