Vitamin B12 gives you energy

In people who are deficient in vitamin B12, tiredness is one of the recognised effects, and treatment corrects the blood changes that underlie it. In people whose B12 status is adequate, controlled research has not shown that extra B12 raises energy. In the trial closest to that everyday question, 95 tired adults with normal B12 levels, all with irritable bowel syndrome or inflammatory bowel disease, took 1,000 micrograms of oral B12 a day or a placebo for 8 weeks, and their fatigue scores fell by about the same amount in both groups. A 2021 systematic review of randomized, placebo-controlled trials in people without overt deficiency, which did not include that trial, found only one trial that measured fatigue, too little to analyse. In the largest placebo-controlled trial found that reported a vitality score for B12 by itself, 140 adults with a median age of about 75 and an early blood sign of low B12 received B12 or saline injections, and their vitality scores three months later did not differ. A smaller trial, in 15 people with chronic fatigue syndrome, found no difference between a B12-containing injection and placebo.

Most of the results that point toward a benefit come from small trials with indirect or partial results, or from studies without a comparison group. In a 2026 trial, 18 trained male cyclists with normal B12 took 1 mg of methylcobalamin under the tongue or a placebo for 3 days each, in random order; with B12, peak sprint power was about 4 percent higher, power fell off less across five sprints, and a reaction test was completed about 5 percent faster. The trial was paid for by HealthTech BioActives, the company that makes the methylcobalamin ingredient tested, it was registered after it began, and its report gives peak power and the fall-off but not the average power it had also registered as an outcome. A 1973 crossover trial in 28 people complaining of tiredness found better general well-being with hydroxocobalamin injections than with placebo only among those who had placebo first, and no difference among those who had B12 first; the 2021 review excluded it as at high risk of bias. In an open study of 51 adults with ME/CFS given high-dose B12 nasal drops, fatigue scores improved in the 34 who described themselves as responders and not in the other 17. A review of 2,142 medical records from people with fibromyalgia found fatigue more common at lower B12 levels. Neither design can separate an effect of B12 from expectation or from other differences between people. In the 140-person trial, one of eight health scales, self-rated general health, improved slightly more with B12 than with placebo, by less than half the change the trial was designed to detect, and its authors hesitated to conclude that B12 improved well-being. In the 95-person trial, the abstract reports a small improvement on one fatigue subscale, motivation, beside no difference on the main fatigue scale. The NIH Office of Dietary Supplements reports that B12 supplementation appears to have no effect on performance or endurance in people with sufficient B12 status.

Low B12 has several known causes: reduced absorption from food, which becomes more common with age; pernicious anaemia, in which the protein needed to absorb B12 is missing; surgery on the stomach or intestines; long-term use of metformin or of acid-suppressing drugs such as proton pump inhibitors; and diets low in animal foods, which are the only natural source. Because the body stores 1 to 5 mg, symptoms can take years to appear, and nerve symptoms such as numbness and tingling can occur without anaemia. Fatigue has many causes, and B12 status is measured by a blood test.

Even in people who are deficient, the treatment trials have mostly measured blood levels rather than how tired people feel. A 2018 Cochrane review found three randomized trials, 153 participants in all, comparing oral B12 with injections. Oral doses of 1,000 micrograms a day produced blood levels similar to injections, and 2,000 micrograms a day produced higher ones, but no trial reported symptoms, quality of life or acceptability. A larger trial published in 2020 followed 283 deficient adults aged 65 or over for a year on oral or injected B12. Blood levels were normal in more than 90 percent of both groups at eight weeks, and in 74 percent of the oral group and 80 percent of the injection group at one year. Quality-of-life scores stayed close to where they began in both groups. Everyone in that trial received B12 and it reported no fatigue score, so it does not show what treatment did to tiredness. These results concern energy and well-being. The blood and nerve effects of deficiency described above are a separate matter.

Most of the fatigue studies used forms unlike an everyday tablet: 1,000 microgram injections once a week for four weeks in the 140-person trial, 5 mg hydroxocobalamin injections twice a week for two weeks in the 1973 trial, injections combining B12 with folic acid and liver extract in the 1989 trial, and 10,000 micrograms of hydroxocobalamin as nasal drops twice a week in the ME/CFS study, where median blood B12 was 328 pmol/L before treatment. The exceptions are the 95-person trial, which used a daily 1,000 microgram tablet, and the cyclist trial, which used a 1 mg tablet held under the tongue for 3 days. No tolerable upper intake level has been set for B12, because of its low potential for toxicity. The US prescribing label for B12 injections lists them for deficiency caused by poor absorption, does not mention fatigue or energy, and warns of rare severe allergic reactions after injection.

Registered research on B12 by itself and energy is currently small and aimed at athletes: randomized crossover trials in 18 to 20 trained men testing a few days of B12 on sprint and reaction tests. The first, in 18 cyclists, was published in a journal in May 2026 and is described above; the second, in 20 men, had posted no results when its registry record was checked on 12 October 2026.