Are vitamin B12 tablets as good as injections?

What the studies found

• A 2018 Cochrane review included 3 trials with 153 people with a B12 deficiency. Tablets of 1,000 to 2,000 µg a day brought blood B12 back to normal about as well as injections over 3 to 4 months. In one trial, 2,000 µg a day gave a blood level 680 pg/mL higher than injections. Tablets also cost less. The quality of the evidence was low.^[1]
• A 2024 network meta-analysis of 13 studies with 4,275 patients compared tablets, tablets under the tongue and injections. All three raised B12, and the differences between them were not significant.^[3]
• In a 1998 US trial, 38 people with a new deficiency got 2 mg (2,000 µg) of cyanocobalamin by mouth every day, or 1 mg by injection on 9 days over 3 months. Blood counts and nerve problems got better just as fast in both groups. After 4 months, blood B12 was 1,005 pg/mL in the tablet group and 325 pg/mL in the injection group.^[4]

Why high-dose tablets work

Intrinsic factor, the protein that carries B12 into the body, takes only 1 to 2 µg at a time. With bigger doses, a small share still gets in: about 1.3% of a 1,000 µg dose.^[5] That is about 13 µg, more than the adult need of 2.4 µg a day.^[5] Supplements that contain only B12 usually have 500 to 1,000 µg.^[5]

When injections are still used

NICE, the UK guideline body, advises lifelong injections for some causes of deficiency. These are autoimmune gastritis, removal of the whole stomach, and removal of the last part of the small intestine.^[2] For other gut causes, such as coeliac disease or some weight-loss operations, NICE advises doctors to consider injections instead of tablets. When tablets are used, the dose is at least 1 mg a day.^[2] When the cause is diet or unknown, NICE advises to consider tablets first. Injections are considered when symptoms may get worse fast, or when a person may have trouble taking tablets regularly.^[2] If symptoms do not improve enough on tablets, NICE advises a higher dose by mouth, or a switch to injections.^[2]

Under the tongue and nasal gel

Studies suggest that tablets held under the tongue (sublingual) work as well as tablets that are swallowed.^[5] A prescription nasal gel also appears to raise B12 levels.^[5]

What is still unknown

The trials in the Cochrane review were small, and none of them reported symptoms or quality of life.^[1] NICE advises a first check-up 3 months after treatment starts, to see whether symptoms have improved.^[2]