Vitamin D Dose and Upper Limits: Micrograms, IU and Ceilings

Two numbers cause most of the confusion about vitamin D: the amount to aim for and the amount not to exceed. This guide sets out what the NHS, the government's Scientific Advisory Committee on Nutrition (SACN) and the Food Standards Agency (FSA) say about both, and how to convert between the units on a label. It is general information for healthy adults, not medical advice. For choosing a product, see our buying guide.

Micrograms, IU and the label percentage

The NHS and SACN both give 1 microgram (µg) of vitamin D as 40 international units (IU). The conversions you will meet most often are:

• 5 µg = 200 IU
• 10 µg = 400 IU
• 20 µg = 800 IU
• 25 µg = 1,000 IU
• 50 µg = 2,000 IU
• 100 µg = 4,000 IU

In England the Food Supplements (England) Regulations 2003 require the amount per recommended daily portion in numbers, plus a percentage of the reference value, which for vitamin D is 5 µg. So 5 µg shows as 100%, 10 µg as 200% and 25 µg as 500%. The percentage is a label convention, not a target.

What the advice says

• Adults and children from age one: the NHS says 10 µg a day, which includes pregnant and breastfeeding women and people at risk of deficiency.
• Babies up to one year: 8.5 to 10 µg a day.

SACN's 2016 report frames the same figure as a Reference Nutrient Intake (RNI) of 10 µg a day for everyone aged four and over, with "safe intakes" for younger children because there was too little data for an RNI. It describes the RNI as the average amount that about 97.5% of the population need to keep blood vitamin D at or above 25 nmol/L when sunlight is minimal, counting vitamin D from all sources, and as an average over a period such as a week. A missed day is therefore not a problem, but the NHS says on its pages about vitamin D medicine never to take two doses at once to make up for a forgotten one.

Our guide to who needs a vitamin D supplement sets out the months and groups.

The upper limits

The NHS says: do not take more than 100 µg (4,000 IU) of vitamin D a day, as it could be harmful. This applies to adults, including pregnant and breastfeeding women and older people, and to children aged 11 to 17. Children aged 1 to 10 should not have more than 50 µg (2,000 IU) a day, and infants under 12 months not more than 25 µg (1,000 IU).

Behind these figures, SACN's report records that the European Food Safety Authority set the limits in 2012 using high blood calcium (hypercalcaemia) as the key sign of harm, and that the UK Committee on Toxicity agreed they were appropriate. The limits apply to total intake and do not separate food from supplements, because food contributes little at those levels. SACN also warns that they may not protect people with conditions that predispose to high blood calcium, including normocalcaemic hyperparathyroidism, sarcoidosis, tuberculosis and certain genetic conditions.

SACN's report also records an earlier, lower UK figure. In 2003 an expert group found too little data for a safe upper level and set a guidance level of 25 µg a day for supplemental vitamin D, on top of food. The ceiling is a limit, not a goal.

Why more is not the aim

The NHS says 10 µg a day is enough for most people who choose to take a supplement, and warns that taking too much for a long time can lead to a build-up of calcium in the body (hypercalcaemia), which may weaken the bones and harm the kidneys and heart. The FSA says not to take high-dose supplements unless a GP or healthcare specialist advises it, so that they can check the need and advise how long to take them.

A product being on sale does not make its dose sensible. SACN noted in 2016 that most multivitamins carried 5 µg a day, while supplements of up to 250 µg a daily dose were available, well above the adult ceiling.

Reading a high-strength label

• 25 µg (1,000 IU) is 2.5 times the NHS amount and well below the adult ceiling. It equals the ceiling for infants under 12 months, so it is not a product for babies.
• 50 µg (2,000 IU) is half the adult ceiling and is the ceiling for children aged 1 to 10.
• 100 µg (4,000 IU) is the adult ceiling itself. Do not choose a product that reaches it without a doctor's advice.
• Directions that allow two. If the label permits one to two tablets a day, the larger figure is your maximum, so a 10 µg tablet taken twice supplies 20 µg.

Count every source. The BDA says not to take other supplements that contain vitamin D, such as cod liver oil or multivitamins, with a vitamin D supplement, and the NHS says one supplement is enough for a child as long as it contains the recommended amount. Here is a search for vitamin D3 400 IU on Amazon (opens in a new tab) and one for vitamin D3 1000 IU, 25 microgram, on Amazon (opens in a new tab); the label tells you what a portion supplies.

Prescribed doses are different

For people diagnosed with deficiency, a doctor may prescribe a higher-strength course. The NHS's page on vitamin D medicine describes loading doses of up to 300,000 IU taken over several weeks and a maintenance dose of 800 to 2,000 IU a day, and says that adults maintaining their levels can take 400 IU a day. Those are treatment under a doctor's instruction, not a guide for a shop supplement. If your doctor recommends a different amount, the NHS says to follow their advice.

Signs of too much

The NHS says to contact 111 for advice if an adult has taken more than 4,000 IU in a day, a child aged 1 to 10 has had more than 2,000 IU, or a child under 12 months has had more than 1,000 IU. It also lists symptoms that should prompt you to tell your doctor or contact 111, including feeling weak, pain in muscles or bones, passing urine more often, thirst, tiredness, feeling or being sick, constipation, loss of appetite, headaches, confusion and pain in the abdomen, which could be a sign of kidney stones.

Making a daily habit work

Divide the units in a pack by the units in one portion to see how long it lasts. For tablets and capsules, NHS advice for its vitamin D medicine is to take them with your main meal of the day, and the Vegan Society notes that vitamin D is fat-soluble, so a meal or snack containing fat may help absorption. One small study of 17 patients, with no comparison group, reported higher blood levels after they took their usual supplement with the largest meal, so treat that as a hint, not a rule. A weekly organiser can help: weekly pill organisers on Amazon (opens in a new tab) is a search for them.

Who should ask first

Speak to a GP or pharmacist before you start if you are pregnant or breastfeeding, have kidney disease, high blood calcium or sarcoidosis, take medicines such as digoxin, some water tablets or epilepsy medicines, or are choosing for a baby or child. The label is the authority on your product's dose and warnings, and a doctor or pharmacist is the right person to ask about your own health.