Iron deficiency vs B12 deficiency: how to tell which is causing your fatigue

Why this matters

Treating iron deficiency with B12 injections does nothing. Treating B12 deficiency with ferrous sulfate does nothing — worse, it can mask the underlying problem while neurological damage progresses. Getting the differentiation right matters. The good news: it is usually straightforward on a full blood count plus a couple of follow-up tests.

The defining clue — cell size on the FBC

Every full blood count reports the Mean Cell Volume (MCV) of red cells. It is the key differentiator:

• Iron deficiency — microcytic anaemia. Low MCV (typically <80 fL), low haemoglobin, often low MCH (mean cell haemoglobin). The body cannot make enough haemoglobin without iron, so it makes smaller, paler red cells.
• B12 (and folate) deficiency — macrocytic anaemia. High MCV (typically >100 fL), low haemoglobin. DNA synthesis is impaired, so red cell precursors divide poorly and emerge larger than normal.
• Mixed deficiency — the MCV can sit in the normal range despite both deficiencies being present. Vegetarian women with heavy periods are a classic case.

The MCV is the headline number. If your GP has run a full blood count, the MCV will be on the printout.

Symptoms that overlap

Both deficiencies cause:

• Fatigue, particularly on exertion.
• Breathlessness on stairs or climbing hills.
• Pale skin and conjunctivae.
• Brittle nails (more pronounced in iron deficiency, sometimes spoon-shaped — koilonychia).
• Glossitis — sore, smooth tongue.
• Tachycardia or palpitations at rest if severe.

Symptoms that distinguish

Pointers toward B12 deficiency, not iron

• Neurological symptoms — peripheral neuropathy (tingling or numbness in feet and hands), cognitive change, low mood, problems with balance. These are red flags for B12 and need urgent attention even with normal or borderline B12 levels. Pernicious anaemia and dietary deficiency both cause neurology.
• Glossitis with smooth, beefy red tongue.
• Yellow tinge to skin (jaundice) in severe B12 deficiency from haemolysis of fragile red cells.

Pointers toward iron deficiency, not B12

• Pica — craving non-food items like ice, dirt, clay, paper. Highly suggestive of iron deficiency.
• Restless legs syndrome — associated with low ferritin.
• Heavy menstrual bleeding — the single commonest cause in pre-menopausal women.
• Hair shedding — telogen effluvium with low ferritin.

The blood test pathway

Step 1 — Full Blood Count

Always the starting point. Tells you haemoglobin and MCV.

Step 2 — If microcytic (low MCV)

• Ferritin — storage form of iron. Low ferritin is the most sensitive marker of iron deficiency. However, ferritin is an acute-phase reactant and is artificially raised in infection, inflammation, liver disease and chronic illness.
• Transferrin saturation — useful where ferritin is normal but iron deficiency still suspected.
• CRP — to confirm there is no inflammation falsely elevating ferritin.

Step 3 — If macrocytic (high MCV)

• Serum B12 — levels <200 ng/L are typically deficient; 200–350 ng/L is grey zone; >350 is usually adequate but does not rule out functional deficiency.
• Folate — always tested with B12, as deficiencies often co-exist and treating one without the other can worsen the other.
• Intrinsic factor antibodies — if pernicious anaemia is suspected. See our pernicious anaemia guide for the full pathway.
• Methylmalonic acid (MMA) and homocysteine — for functional B12 deficiency where serum B12 is in the grey zone but symptoms suggest deficiency.

Reflex testing

Many labs reflex automatically: if MCV is low, ferritin is run; if MCV is high, B12 and folate are run. We arrange the right panel up front so reflex steps are not needed. The broader reading blood test results guide explains the wider interpretation framework.

When both are present

Mixed iron and B12 deficiency is common in specific groups:

• Vegetarians and vegans — iron from plant sources (non-haem) is less bioavailable; B12 is essentially absent from plant foods. See our vegan B12 guide.
• Post-bariatric patients — reduced absorption surface for both nutrients.
• Inflammatory bowel disease (Crohn's, ulcerative colitis) — disrupts absorption in different parts of the bowel.
• Coeliac disease — villous atrophy reduces nutrient absorption broadly.
• Women with heavy menstrual bleeding — chronic iron loss, often layered on borderline dietary B12 intake.

Causes worth investigating

Iron deficiency causes

• Heavy menstrual bleeding (commonest in pre-menopausal women).
• Occult gastrointestinal bleeding — ulcer, colorectal lesions, hiatus hernia. Iron deficiency in a man, or a post-menopausal woman, warrants a GI workup.
• Malabsorption — coeliac, IBD.
• Dietary insufficiency — less common but possible.

B12 deficiency causes

• Pernicious anaemia — autoimmune loss of intrinsic factor. Most common cause in older adults.
• Dietary — vegans, strict vegetarians.
• Drug-induced — chronic metformin and chronic proton pump inhibitor use both cause it.
• Post-bariatric and gastric surgery — reduced intrinsic factor production.
• Terminal ileal disease — the absorption site for B12.

Treatment differs completely

Iron

Oral ferrous sulfate or ferrous fumarate, typically for 3–6 months to replenish stores after haemoglobin normalises. Take on an empty stomach with vitamin C if tolerated. Side effects include nausea, constipation, dark stools. Intravenous iron is an option where oral is not tolerated or not effective.

B12

For pernicious anaemia: intramuscular hydroxocobalamin loading doses (typically three doses in the first week) followed by maintenance every 2–3 months for life. For dietary deficiency: oral cyanocobalamin is usually sufficient. See hydroxocobalamin vs cyanocobalamin for the choice between the two preparations, and B12 deficiency symptoms for the wider clinical picture.

The practical pharmacist conversation

Patients often arrive with a partial picture — a GP printout, some symptoms, a sense that something is not right. We work through:

• What the existing results show — and what they do not.
• What follow-up testing makes sense.
• Whether your GP investigations have followed NICE pathways.
• Whether private testing accelerates the picture or duplicates NHS work.

For background on private vs NHS testing, see our private blood tests vs NHS guide.

Local pages

B12 services are available at Empire Pharmacy (GPhC premises 1123966), 122 Seymour Grove, Old Trafford. Patients book from Manchester, B12 Manchester, Sale, B12 Altrincham and blood tests Altrincham. Brand pages: blood tests and vitamin B12.

If you suspect either deficiency, call 0161 258 6149 or book via our booking page. We will arrange the right tests and read them properly.