The B12-folate interaction is one of the most important pharmacological interactions in clinical medicine — the 'folate trap' or 'methylfolate trap' means that B12 deficiency causes secondary intracellular folate deficiency. When folic acid is given to a patient with B12 deficiency, it partially corrects the haematological abnormalities (the blood count and MCV improve) but does nothing to address the neurological B12 deficiency. The neurological damage continues, and in fact appears to accelerate in some studies when folate is given without B12 — possibly because the increased DNA synthesis demand from haematopoietic recovery increases B12 utilisation, depleting the limited B12 stores further and worsening the neurological picture. The rule is absolute: NEVER start folic acid supplementation for macrocytic anaemia or folate deficiency without first checking and acting on B12. If both are deficient: treat B12 first, or treat both simultaneously. Neurological damage from SACD is irreversible once established — prevention by correct sequencing of treatment is the only protection.

Coeliac disease is the single most important treatable cause of folate deficiency to exclude in any patient presenting with low folate and macrocytic anaemia — approximately 10-15% of new coeliac diagnoses are made on the back of investigating folate or iron deficiency anaemia in primary care. The typical coeliac anaemia presentation: combined iron deficiency + folate deficiency (dual deficiency in the proximal small bowel — iron and folate both absorbed in the duodenum and proximal jejunum); this creates a 'dimorphic' blood picture (microcytic + macrocytic populations) where the MCV may appear normal despite significant dual deficiency. Anti-tTG IgA (with total IgA to exclude IgA deficiency) is the first-line coeliac test. IgA deficiency (affecting approximately 2-3% of coeliac patients) causes false-negative anti-tTG IgA — if total IgA is low, request anti-tTG IgG or anti-DGP IgG instead.

Red cell folate versus serum folate provide complementary information — serum folate (the standard test in most UK labs) reflects recent dietary intake over the preceding days, meaning a single good dietary intake before the blood test can give a falsely reassuring normal serum folate even in a patient with chronic deficiency. Red cell folate (RCF) reflects the folate status averaged over the lifespan of the red blood cell (approximately 120 days) and is therefore a more reliable indicator of chronic folate status. However, red cell folate can also be low in B12 deficiency (folate trap — secondary RCF depletion from B12 deficiency). In clinical practice, the combination of serum folate with homocysteine (elevated in both B12 and folate deficiency) and methylmalonic acid (elevated only in B12 deficiency) provides the most complete metabolic picture. Most UK labs now report serum folate rather than red cell folate routinely — interpret with the clinical context.

The neural tube defect (NTD) high-dose folic acid (5 mg) prescription for high-risk groups is frequently under-prescribed in UK primary care — a Public Health England audit (2019) found that only approximately 30-40% of women with known risk factors received the recommended high-dose folic acid preconceptionally. The risk groups requiring 5 mg (not 400 mcg) are: previous NTD-affected pregnancy (approximately 5% recurrence risk; 5 mg reduces recurrence by approximately 70%), women on enzyme-inducing antiepileptic drugs (valproate, carbamazepine, phenytoin, topiramate), pre-existing type 1 or type 2 diabetes (NTD risk 5-10x baseline due to hyperglycaemia-related embryopathy), BMI ≥30 (folate is fat-soluble and distributed into adipose tissue, reducing bioavailability in the first trimester), and malabsorption conditions (coeliac disease, IBD, bariatric surgery). The 5 mg dose is prescription-only — GPs prescribing preconception care for women with any of these risk factors must ensure the higher dose is prescribed and documented.

The folic acid duration of treatment depends critically on whether the cause has been corrected — for reversible causes of folate deficiency (recent dietary inadequacy, short-course trimethoprim, acute alcohol excess), folic acid 5 mg OD for 4 months is the standard course. This duration reflects the time needed to replete red cell folate stores (which have a 120-day lifespan) and correct the megaloblastic anaemia completely. However, for ongoing causes of increased demand or persistent malabsorption, long-term supplementation is required: lifelong for haemolytic anaemias (sickle cell, hereditary spherocytosis, thalassaemia), and until gluten-free diet normalises absorption for coeliac disease (typically 6-12 months minimum). The most common prescribing error is stopping folic acid at 4 months in a patient with coeliac disease who has not yet had mucosal recovery — documenting the reason for supplementation and the expected duration is essential.

The methotrexate folate supplementation day-timing principle is clinically important and frequently misunderstood — folic acid should NOT be taken on the same day as methotrexate. The reason: while folic acid supplementation does not meaningfully reduce MTX's therapeutic antifolate effect on actively inflamed synovial tissue (at the doses used for RA/psoriasis), taking folic acid simultaneously with MTX theoretically maximises the competitive inhibition of DHFR by giving both at peak plasma concentrations simultaneously. The convention of taking folic acid the day AFTER MTX is therefore both pharmacologically logical and practically simpler for patients to remember ('MTX Monday, folic acid Tuesday'). Some rheumatologists now advocate folic acid 5 mg daily on all 6 non-MTX days for patients with persistent GI side effects — this approach is well-evidenced and is acceptable in shared care.

Coeliac disease-related osteoporosis is one of the most important and frequently underappreciated long-term complications — the combination of calcium malabsorption (from small bowel damage), vitamin D malabsorption (fat-soluble vitamin), and ongoing systemic inflammation (before GFD) causes accelerated bone density loss. Approximately 30-40% of untreated coeliac patients have osteoporosis at diagnosis (T-score ≤-2.5) and a much higher proportion have osteopenia. DEXA scanning at diagnosis is recommended by the BSG for: all adults with coeliac disease diagnosed after the age of 55, any patient with documented nutritional deficiencies, and any patient with fracture risk factors. After starting GFD: bone density typically improves by approximately 5% per year in the first 2 years as calcium absorption normalises. GPs managing newly diagnosed coeliac patients should request DEXA and vitamin D level at diagnosis, and ensure annual vitamin D monitoring thereafter.

The UK Valproate Pregnancy Prevention Programme (PPP) is a critical regulatory safety framework that every GP prescribing valproate must be enrolled in — sodium valproate causes NTDs in approximately 1-2% of exposed pregnancies and severe neurodevelopmental harm (autism, intellectual disability) in approximately 30-40% of children exposed in utero. The MHRA mandated the PPP in 2018: all women and girls of childbearing potential prescribed valproate must: enrol in the PPP; have an annual benefit-risk review with the specialist (neurologist or psychiatrist); be informed of the risks verbally and in writing at every prescription; use highly effective contraception; and have a PNPC (Pregnancy Prevention Programme Card) signed at each annual review. GPs prescribing repeat valproate in primary care should confirm: PPP card is signed and current, effective contraception is confirmed, and annual specialist review is documented. GPs who prescribe valproate to women of childbearing age without PPP compliance are in breach of MHRA regulatory requirements.

The SACD (subacute combined degeneration of the cord) risk from folate-before-B12 is the central safety message of this algorithm and deserves reinforcement at every folate deficiency consultation — the clinical scenario that kills: a patient presents with macrocytic anaemia and low folate is checked. B12 is not checked. Folic acid 5 mg is prescribed. The MCV normalises, the Hb rises, and the GP is satisfied. Three months later, the patient presents with progressive unsteady gait, tingling in hands and feet, and difficulty walking. B12 is now checked — it is profoundly deficient. The neurological damage from SACD, which would have been prevented by checking B12 at the outset and treating it, is now established and partially irreversible. This scenario is preventable by a single rule: NEVER prescribe folic acid for macrocytic anaemia or folate deficiency without checking B12 simultaneously.