What's the difference between D2 and D3 and which should I recommend?

How it works

D3 is the form humans synthesise from sunlight (UVB conversion of 7-dehydrocholesterol in skin). D2 is plant-derived from UV-irradiated fungi (most often yeast or mushrooms). Lichen-derived vegan D3 is now widely available in the UK and other markets, manufactured from lichen species naturally producing high levels of vitamin D3.

Effective dose

Therapeutic loading regimens for confirmed deficiency are typically specified as D3 in UK regional NHS formularies (e.g. 50000 IU oral colecalciferol weekly for 6-8 weeks); equivalent D2 regimens exist but are less commonly used. The Tripkovic 2017 D2-D3 Study used 15 mcg/day (600 IU) over 12 weeks; D3 raised 25(OH)D approximately 2x more than D2 at this dose, but the ratio varies by dose and frequency.

Forms compared

Both colecalciferol (D3) and ergocalciferol (D2) appear on UK NHS regional primary care formularies and are clinically acceptable. Practical reasons D3 has become the more commonly prescribed form: stronger and more recent efficacy evidence; greater sustained rise in 25(OH)D; alignment with the form humans produce endogenously; wider OTC availability of higher-strength D3 products. For deficiency repletion in the UK, colecalciferol is the typical first-line.

Timing

Tripkovic 2017 measured 25(OH)D at 12 weeks. Bolus dosing (e.g. 50000 IU weekly for 6-8 weeks loading) shows a greater D3 vs D2 advantage than daily dosing per Balachandar 2021 systematic review. Annual or quarterly bolus regimens are avoided in current UK practice (separate harm signals; see entry d0048a63).

Safety profile

Form choice (D2 vs D3) does not change the toxicity profile substantively; toxicity is dose-related and 25(OH)D-driven. Hypercalcaemia clinical features (nausea, weakness, polyuria, kidney impairment) and laboratory hallmarks (elevated calcium with elevated phosphate, suppressed PTH, hypercalciuria) apply to either form. Higher-risk populations (sarcoidosis, primary hyperparathyroidism, CYP24A1 variants, advanced kidney disease) warrant clinical input regardless of form.

Special populations

Older adults: D3 produces a greater 25(OH)D rise per IU than D2; preferred form for repletion. Darker-skinned individuals living in the UK (entry 16d95e91 covers this in detail): same form preference applies; the relevant difference is dose and seasonality, not form. Anticonvulsant users: may need higher dose of either form due to hepatic enzyme induction; D3 preferred where stronger evidence exists.

Interactions

Anticonvulsants (phenytoin, carbamazepine, phenobarbital): may increase vitamin D requirement via hepatic enzyme induction; both D2 and D3 affected. Thiazide diuretics: theoretical hypercalcaemia risk at high vitamin D doses. Glucocorticoids long-term: may increase vitamin D requirement. Calcium supplements: high-dose vitamin D combined with high-dose calcium increases hypercalcaemia and stone risk.

Guideline positions

Balachandar 2021 (Nutrients 13(10):3328) updated systematic review confirming D3 superiority; difference was smaller in daily-dosing trials than in bolus-dosing trials. Heaney/Armas 2004 (J Clin Endocrinol Metab 89(11):5387-5391, PMID 15531486) was the foundational early evidence showing D3 produces greater and longer-lasting 25(OH)D rise after a single bolus. Holick 2011 (PMID 18089691) early equivalency claim now outweighed by larger and more recent trials. Endocrine Society 2024 clinical practice guideline (Demay 2024 PMID 38828931) recommends against routine 25(OH)D testing in healthy adults at standard prophylactic doses.

Practical framework

For deficiency repletion in the UK, colecalciferol (D3) is the typical first-line. Lichen-derived vegan D3 is widely available; veganism does not require accepting lower efficacy. NHS upper daily limits apply to both forms; safety considerations sit with dose, not form. This is a summary of published research, not personal health advice. Discuss any health or supplement decisions with a qualified healthcare professional, particularly during ongoing care, pregnancy, or with chronic conditions.

Common misconceptions

Claim: vitamin D should always be paired with K2 to direct calcium into bone. Vitamin K2 has its own evidence base for surrogate cardiovascular markers (Knapen 2015 Thromb Haemost arterial stiffness; AVADEC 2022 Diederichsen Circulation negative for aortic valve calcification progression in established disease) but does not establish that K2 is required to make D3 supplementation safe; D3 toxicity is dose-related, not K2-deficiency-driven. Entry dde5d38f covers the K2 evidence in detail.

Claim: D2 catches up to D3 at higher doses. The Tripkovic and Balachandar evidence shows D3 superiority at clinically relevant daily doses; the gap is larger in bolus dosing.