Vitamin D3. Most people in the UK need some over winter, and a minority need considerably more than that.
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Colecalciferol is vitamin D3 — the same form your skin makes from sunlight, and generally better absorbed and more effective at raising blood levels than the D2 form.
Despite the name it behaves more like a hormone than a vitamin. It regulates calcium and phosphate absorption, which is why it matters for bone, muscle and immune function.
It is available over the counter and on prescription at higher doses.
Usually once daily, though weekly and monthly higher-dose regimens are also used.
Take it with the fattiest meal of the day. Vitamin D is fat-soluble and absorption is meaningfully better with food containing fat — taken on an empty stomach, a good proportion is wasted.
UK guidance is that everyone should consider 10 micrograms (400 IU) daily between October and early March, because at this latitude the sun is not strong enough to produce vitamin D through the winter.
Some groups are advised to take it year-round: people with darker skin, those who cover up or are rarely outdoors, and anyone over 65.
At normal doses, vitamin D is very well tolerated and side effects are uncommon.
The risk is at the other end — excessive supplementation. Vitamin D is fat-soluble and accumulates, unlike the water-soluble vitamins that are simply excreted. Very high doses over time raise calcium, causing nausea, constipation, excessive thirst, frequent urination, confusion and, over time, kidney stones and kidney damage.
This is genuinely rare and essentially never happens at recommended doses. It happens with high-strength online supplements taken indefinitely without testing, which is now a realistic scenario rather than a theoretical one.
More vitamin D is not better. Correcting a deficiency helps; exceeding the normal range does not confer additional benefit.
Interactions: thiazide diuretics (which raise calcium), digoxin, some anti-epileptics and steroids which increase vitamin D breakdown, and orlistat which reduces absorption of fat-soluble vitamins.
Monitoring: most people supplementing at standard doses need none. A vitamin D level is worth checking where there are symptoms, where deficiency is being treated at high dose, or where an underlying condition affects handling. Calcium should be checked in anyone on long-term high-dose treatment.
Retesting a level immediately after starting is not useful — allow at least three months.
Yes, though standard-dose vitamin D is inexpensive and available in any supermarket, so paying for a consultation to obtain it makes little sense and we would say so.
A consultation earns its keep when the question is bigger than the supplement: persistent fatigue, widespread aches or muscle weakness where vitamin D is one of several things worth checking; bone health assessment where DEXA scanning may be warranted; or deficiency that keeps recurring, which points to malabsorption and warrants coeliac screening.
Worth saying plainly: vitamin D is often blamed for symptoms it is not causing. Correcting a genuine deficiency helps; taking it for fatigue with a normal level does not, and treating it as the answer can delay finding the actual cause. Thyroid disease, iron deficiency and B12 deficiency all present the same way.
This page is information, not an offer to supply.
Vitamin D is the cheapest thing on this website, and it is worth saying plainly: do not pay a doctor to obtain it.
A year's supply of 10 microgram (400 IU) daily tablets costs a few pounds from any supermarket. Own-brand is identical to branded.
Everyone in the UK is advised to consider 10 micrograms daily through autumn and winter, because between October and March the sunlight here is too weak to produce any.
Some groups are advised to take it all year round — people with darker skin, anyone who covers up outdoors, people who are housebound, and children under five.
Usually not. For most people the sensible approach is simply to take the recommended dose rather than measure first.
Testing is worth paying for when there are symptoms that need explaining — bone pain, muscle weakness, fatigue being investigated — or where a malabsorption condition makes deficiency likely.
Vitamin D levels fall again once you stop, particularly through a British winter. There is no lasting reservoir beyond a few months.
For most people it is a small ongoing habit rather than a course — which is part of why it should be cheap.
Where deficiency has been treated with a high loading dose, that is followed by an ongoing maintenance dose rather than stopping altogether.
Stopping completely after loading is a common error, and the level simply falls back over the following months.
If a level is rechecked, wait at least three months. Testing sooner reflects the recent dose rather than your true status.
Most people who are simply taking the recommended maintenance dose do not need retesting at all.
Colecalciferol is D3 and raises blood levels more effectively than ergocalciferol, which is D2.
D3 is usually derived from lanolin, so vegans should look for lichen-derived D3 or use D2 — a detail rarely mentioned on the packet.
Sarcoidosis, kidney stones, high blood calcium and significant kidney disease all change how vitamin D should be given, and are not situations for self-supplementation at high doses.
Through the UK autumn and winter, almost certainly yes at the standard low dose. Sunlight here is too weak from October to March to produce any.
Some groups need it year-round, including people with darker skin and anyone who covers up or is rarely outdoors.
10 micrograms — 400 IU — daily is the standard recommendation.
Higher treatment doses are used for proven deficiency, and those should be taken on advice rather than chosen off a shelf.
Usually not. The standard dose is safe without testing.
Testing makes sense when there are symptoms to explain or a condition that impairs absorption — not as routine.
Yes. Very high doses over time raise blood calcium, causing nausea, thirst, confusion and kidney stones.
The harm comes from high-dose products taken long term, not from the standard dose.
If you are genuinely deficient, correcting it can help — but vitamin D is blamed for a great deal of fatigue it did not cause.
Low iron, low B12, thyroid disease, sleep apnoea and depression are all commoner explanations, and all are worth checking.
Yes — it is fat-soluble and absorbed better with a meal containing some fat.
Taken with a glass of water on an empty stomach, less gets in.
Daily is generally preferred and better tolerated. Weekly or monthly dosing is used for convenience where daily is not realistic.
Most D3 comes from lanolin, so no — look for lichen-derived D3, or use D2.
This is rarely stated clearly on packaging, and is worth checking.
The evidence suggests a modest reduction in respiratory infections, mainly in people who were deficient to begin with.
It is not the immune treatment it is sometimes marketed as — correcting a deficiency helps; taking more than you need does not.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 24, 2026
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