Vitamin D (colecalciferol): supplements, loading doses and safety in the UK

Colecalciferol is vitamin D3. The NHS explains that it helps the body take in more calcium and phosphorus from the food you eat and helps the kidneys keep calcium rather than passing it out in urine. It is converted in the liver and kidneys into calcitriol, the form of vitamin D the body can use, which is why it does not work straight away.

We usually get vitamin D through exposure to sunlight and diet. The NHS says not having enough can lead to bone conditions such as rickets and osteomalacia in children and osteoporosis in adults, and that long-term deficiency can cause bone loss, muscle weakness, falls and fractures in older people.

Several forms of vitamin D exist. The NHS names alfacalcidol, calcitriol and ergocalciferol, and says most vitamin D that you can buy is colecalciferol or ergocalciferol. Calcitriol and alfacalcidol work faster and are used in conditions such as chronic kidney disease; calcipotriol is a separate, similar-sounding medicine used on the skin in plaque psoriasis.

The NHS states that everyone can benefit from a daily vitamin D supplement. Adults who are not at risk of deficiency should consider taking 10 micrograms daily during autumn and winter. Adults who are at risk should take 10 micrograms throughout the year.

It is recommended if you have a deficiency, or to prevent one, if you are not often outdoors (for example if you are housebound or live in an institution such as a care home), are pregnant or breastfeeding, usually wear clothes that cover most of your skin outdoors, have dark skin, which may not make enough vitamin D from sunlight, are 1 to 4 years old, or are a breastfed baby or have less than 500 ml of infant formula a day.

The NHS adds that you may be at higher risk if you are 65 or over, have a malabsorption disorder, have had weight loss surgery, have severe liver disease or end-stage chronic kidney disease, take certain medicines that can affect vitamin D levels, or have been advised to lose weight. Vitamin D deficiency is the most common cause of rickets, so a child with low levels may be prescribed colecalciferol.

To find out whether levels are low, a doctor will arrange a blood test. If your levels are fine or you want to prevent deficiency, lower-dose colecalciferol can be bought from pharmacies and shops.

Doses are measured in international units (IU); the NHS says 1 microgram of vitamin D is the same as 40 IU. A loading dose is a high-strength course, usually taken for a few weeks, to raise low levels quickly. Lower doses are taken to maintain levels or prevent deficiency.

For adults with severe deficiency, the NHS says a doctor may prescribe a loading dose of up to 300,000 IU, taken for example as 50,000 IU once a week for 6 weeks, 40,000 IU once a week for 7 weeks, 1,000 IU four times a day for 10 weeks or 800 IU five times a day for 10 weeks. A maintenance dose afterwards may be 800 IU to 2,000 IU a day, and lower-dose products allow 400 IU a day for adults maintaining levels.

For children, the NHS gives usual loading doses by age: 1 to 5 months 3,000 IU daily, 6 months to 11 years 6,000 IU daily and 12 to 17 years 10,000 IU daily, each for 8 to 12 weeks, followed by a maintenance dose of 400 IU to 600 IU daily if diet changes are not enough. To prevent rickets it gives 340 to 400 IU a day from birth to 1 year and 400 IU a day from 1 to 4 years. These figures are not a substitute for your own prescription.

Capsules and tablets should be swallowed whole with water, with your main meal of the day. Chewable tablets are chewed, dissolvable tablets are dropped into a glass of water and drunk straight away, and syrup is measured with a syringe. Adults on a loading dose usually take it daily or weekly for 6 to 10 weeks, and maintenance or prevention doses are usually taken long term.

If you stop colecalciferol, the NHS says your symptoms may come back, bone health may be affected and calcium levels could fall, causing bone pain or muscle weakness. Capsules may contain gelatine, which may matter for vegetarian, vegan, halal or kosher diets.

The NHS says taking low doses daily is safe and does not normally cause side effects. Common effects include stomach pain, feeling sick and headache. Call a doctor or NHS 111 if you need to pee more often, have swelling, a high temperature with sharp pain in your side or lower back, which could be early kidney stones, or are sick, confused, have bone or joint pain or an irregular heartbeat, which could be signs of too much calcium.

Taking too much over a long period can cause a skin rash and high calcium levels, which can cause stomach pain, weaken bones and damage the kidneys and heart. Your doctor will arrange a blood test to check calcium levels and may ask you to stop until they are normal.

Unless you are on a prescribed loading dose, an extra dose is unlikely to harm you. The NHS says to contact 111 for advice if an adult has taken more than 4,000 IU in one day, a child aged 1 to 10 years more than 2,000 IU in a day, or a child under 12 months more than 1,000 IU in a day. Symptoms of too much include weakness, muscle or bone pain, passing urine more often, thirst, tiredness, being sick, constipation, loss of appetite, sweating, headache, confusion, drowsiness and abdominal pain.

Tell your doctor before starting if you have had an allergic reaction to colecalciferol, a peanut or soya allergy, high calcium in the blood or urine, kidney problems or kidney stones, high vitamin D levels, sarcoidosis or calcification. A serious allergic reaction is rare but is a 999 emergency.

Most people can make the vitamin D they need from sunlight on the skin outdoors, but we do not always make enough. The NHS says foods that are rich in vitamin D can help, including oily fish such as salmon, sardines, herring and mackerel, red meat, liver, egg yolks and fortified foods such as some fat spreads and breakfast cereals.

Colecalciferol does not affect contraception, driving or cycling, and you can drink alcohol while taking it. There are no foods or drinks you need to avoid.

Symptoms such as bone, muscle or joint pain are likely to improve once vitamin D levels return to normal, usually after a few weeks or months. If symptoms last longer, your doctor may check again.

Lower-dose colecalciferol can be bought from pharmacies and shops, while the NHS says loading doses are a high-strength course that is only available on prescription. The NHS says a doctor will use a blood test to find out whether your levels are low, so it is worth asking your GP before starting a high-strength product.