Executive summary
- Vitamin D deficiency is extremely common in the UK — particularly in winter months, in individuals with darker skin, limited sun exposure, or who are obese or housebound. The UK does not have sufficient UVB sunlight for adequate cutaneous vitamin D synthesis from October to March at any latitude.
- NICE recommends routine supplementation for all adults in at-risk groups without testing, and for all people from October to March if diet and lifestyle cannot reliably provide adequate vitamin D.
- Serum 25-hydroxyvitamin D (25-OHD) is the correct test — do not test 1,25-dihydroxyvitamin D (active form) routinely; it does not reflect body stores and is often misleadingly normal.
- Treatment is safe and straightforward: Cholecalciferol (D3) is preferred over ergocalciferol (D2) for supplementation and treatment due to superior bioavailability and half-life.
Who to test and at-risk groups
- Test serum 25-OHD if: Symptoms suggestive of deficiency (fatigue, bone pain, proximal myopathy, depression, recurrent infections), conditions that increase risk (CKD, malabsorption, chronic liver disease, bariatric surgery, use of enzyme-inducing medications), or osteoporosis/fragility fracture assessment.
- Do not routinely test the general population. Routine supplementation is the more cost-effective strategy for asymptomatic individuals in high-risk groups.
- High-risk groups who should take vitamin D year-round without testing: People who are not exposed to much sun (housebound, care home residents), people with darker skin (South Asian, African, African-Caribbean, Middle Eastern), those who cover their skin for cultural or religious reasons, and all pregnant and breastfeeding women.
- Infants: All breastfed infants (breast milk is low in vitamin D) and formula-fed infants taking <500 ml formula per day should receive vitamin D 8.5–10 micrograms (340–400 IU) daily from birth. This is a national recommendation.
Interpreting serum 25-OHD levels
- Deficiency: <25 nmol/L (<10 ng/mL) — risk of osteomalacia, rickets (children), and severe clinical consequences. Requires treatment with a loading regimen.
- Insufficiency: 25–50 nmol/L (10–20 ng/mL) — suboptimal; supplementation is indicated. A loading regimen may be appropriate if symptoms are present or the patient is high-risk.
- Adequate: ≥50 nmol/L (≥20 ng/mL) — generally sufficient for bone health. Maintenance supplementation may still be appropriate in high-risk groups to maintain this level through winter.
- Toxicity is rare with cholecalciferol supplementation at recommended doses. It typically requires very high daily doses (>250 micrograms / 10,000 IU per day over months). Toxicity causes hypercalcaemia — check calcium if symptoms develop in a patient on high-dose supplementation.
Treatment regimens
- Loading regimen (deficiency <25 nmol/L, or symptomatic insufficiency): A total of approximately 300,000 IU of cholecalciferol given over 6–10 weeks. This can be given as: 50,000 IU weekly for 6 weeks, or 25,000 IU twice weekly for 6 weeks. Check local formulary — preparations include Fultium-D3, InVita D3, and Stexeril. Recheck 25-OHD 3 months after completing loading.
- Maintenance supplementation (after loading, or for insufficiency/prevention): Cholecalciferol 800–1,000 IU (20–25 micrograms) daily. This is appropriate for most adults at risk of deficiency.
- Routine supplementation (prevention in at-risk groups without testing): Cholecalciferol 400 IU (10 micrograms) daily — available OTC, cost-effective, and recommended by NICE for high-risk groups year-round and for all adults October–March.
- Calcium co-supplementation: Routinely combined with vitamin D only if dietary calcium intake is inadequate or in the context of osteoporosis management. Do not add calcium supplementation without dietary assessment in the general population.
Frequently asked questions
Should I retest after completing a loading regimen?
Yes. Recheck serum 25-OHD approximately 3 months after completing the loading course to confirm levels have risen adequately (aim ≥50 nmol/L). Then switch to maintenance supplementation. Routine annual re-testing of 25-OHD in those on stable maintenance is generally not required unless there is a clinical reason.
Which preparation should I prescribe?
Cholecalciferol (vitamin D3) is preferred over ergocalciferol (vitamin D2). Available licensed UK preparations include Fultium-D3 (available as capsules in various strengths), InVita D3 (oral solution), and Stexeril (high-strength capsules). The choice depends on the dose required and patient preference (capsule vs liquid). Many patients obtain adequate maintenance supplementation via OTC preparations.
My patient has sarcoidosis — should I supplement vitamin D?
Exercise caution. Granulomatous diseases (sarcoidosis, tuberculosis, lymphoma) can cause unregulated conversion of 25-OHD to active 1,25-dihydroxyvitamin D by macrophages, leading to hypercalcaemia even at normal 25-OHD levels. Supplementation may worsen hypercalcaemia. Always check serum calcium first; seek specialist advice before starting vitamin D in known sarcoidosis.
A patient says they take 4,000 IU daily — is this safe?
The UK safe upper level for adults is generally considered 4,000 IU (100 micrograms) per day for long-term use, though toxicity at this dose is uncommon in healthy individuals. The NHS guidance for routine supplementation is 400–1,000 IU. Higher doses should be clinically justified, and if a patient is taking very high doses chronically, check serum calcium and 25-OHD.
Transparency
This page is an educational, clinician-written summary of publicly available NICE guidance intended for trained healthcare professionals. It uses original wording (not copied text) and should be used alongside the full NICE source, local pathways, and clinical judgement. Doses provided are for general reference; always check the BNF/SPC.