Cyanocobalamin (vitamin B12): tablets for B12 deficiency and how they differ from injections

Vitamin B12 is needed to make red blood cells that carry oxygen properly and to keep the nervous system healthy. The NHS describes cyanocobalamin as a manufactured form of the vitamin, used to treat and prevent vitamin B12 deficiency anaemia. It is available on prescription as tablets of 50 micrograms and 1,000 micrograms, and can also be bought as supplements in tablets, capsules, mouth sprays and drops.

Cyanocobalamin is different from hydroxocobalamin, the form usually given as an injection in the UK. The NHS says cyanocobalamin injections are generally not available on the NHS, and that if your doctor prescribes B12 injections they will usually give hydroxocobalamin. The NHS says you will usually take tablets every day, although you may need injections at first to boost your levels, and that high-dose tablets are used if you cannot have injections.

The NHS notes that it contains cobalt, so people with a cobalt allergy should avoid it.

You can get vitamin B12 from food or supplements, but there may not be enough in what you eat. The NHS says this can happen if you eat a vegan diet or do not regularly eat meat, fish or dairy products. Good sources include meat such as liver and beef, fish such as salmon, tuna and haddock, shellfish, dairy foods and eggs, and some foods with B12 added such as breakfast cereals, bread, soya foods, non-dairy milks and yeast extract.

Some conditions or treatments stop the body absorbing enough from food. The NHS lists pernicious anaemia, low stomach acid caused by inflammation of the stomach lining or certain medicines, stomach surgery such as gastrectomy, inflammation of the intestines in conditions such as Crohn’s disease or coeliac disease, and an infection in the stomach or intestine.

If the cause is absorption, eating more B12 will not help. The NHS says that if low levels are caused by stomach or bowel problems or surgery you will not absorb enough from food or low-dose supplements of 10 to 100 micrograms, and will need hydroxocobalamin injections, or high-dose cyanocobalamin tablets of 1,000 micrograms if you cannot have injections.

According to the NHS, for adults with diet-related deficiency the usual dose is 50 to 150 micrograms once a day. For adults with deficiency not caused by diet, the usual dose is one to two 1,000 microgram tablets once or twice a day, typically if injections cannot be given. Children may need lower doses. Your doctor or pharmacist advises on the exact dose, which depends on your blood level and the reason it is low.

Swallow prescribed tablets whole with water; do not chew or break them. It is usually best to take them on an empty stomach, at least 2 hours after a meal or snack and at least 30 minutes before eating again. For supplements you buy, follow the packet instructions.

You may have injections at first to boost your levels. The NHS says that if you have hydroxocobalamin injections at the start, the tablets may start to work within a few days; otherwise it can take a few weeks before levels and symptoms such as extreme tiredness begin to improve.

If you forget a dose, take it as soon as you remember unless it is nearly time for the next one, never doubling up. Taking an extra dose is unlikely to harm you, though the NHS says to contact 111 if you have taken more than your usual dose and feel sick, vomit or have diarrhoea or are worried.

You may have regular blood tests to monitor your vitamin B12 level, and your doctor may adjust the dose according to the results. The NHS says your doctor may also ask about symptoms such as tiredness and lack of energy.

Some people can stop when levels are back to normal. However, you may need to take it for the rest of your life if your diet does not have enough B12 or you cannot absorb enough from food. If you stop prescribed tablets, levels may fall and symptoms such as tiredness may return, which can take days, weeks or months depending on your levels, how long you took it and your diet.

Vitamin B12 supplements only improve tiredness or lack of energy if these are caused by low B12 levels. If tiredness continues despite normal levels, it is worth asking your GP to look for other causes.

The NHS adds that drinking too much alcohol regularly can cause folate deficiency anaemia, which can bring tiredness back, so folate matters too.

Most people do not have side effects from cyanocobalamin tablets. Because it contains cobalt, it can cause sensitivity reactions in people with a cobalt allergy, with symptoms starting straight away or up to 3 days after a dose. The NHS says to talk to your doctor or call 111 for an allergic skin reaction such as a rash or hives.

In rare cases a serious allergic reaction (anaphylaxis) can occur, which is a medical emergency: call 999 for swelling of the lips, mouth, throat or tongue, very fast or difficult breathing, a tight throat, blue, grey or pale skin, sudden confusion or dizziness, or fainting.

Cyanocobalamin does not affect contraception, you can drink alcohol with it, and it does not usually affect driving or cycling. Report suspected side effects through the Yellow Card scheme.

On the NHS, B12 deficiency is diagnosed by blood tests and treated with tablets or injections according to the cause. The NHS says there is no noticeable difference in how much B12 your body can absorb from different types of supplement, such as chewable, slow-release or under-the-tongue tablets, capsules, sprays or drops, so no supplement is more effective than another.