Does taking vitamin B12 give you more energy?

Short answer

Not unless you are low. Giving 95 tired people 1,000 µg a day for 8 weeks did not reduce fatigue compared with a fake pill. People who ran low from a vegan diet or long-term metformin brought their levels back up with oral B12.

Does taking vitamin B12 give you more energy?
'B12 → more energy production → more energy' — does this formula hold?
B12 is a staple of fatigue tonics and 'energy shots'. The logic: your body uses it to make energy, so more should mean more pep. We checked trials that compared it with a fake pill.
With normal levels, fatigue did not drop
1,000 µg a day for 8 weeks: no different from a fake pill
95 people with normal B12 levels but severe fatigue took 1,000 µg a day or a fake pill for 8 weeks. Fatigue scores fell 8.1 points with B12 and 8.3 points with the fake pill: no difference. Mood and quality of life did not change either.
A shot did no better than a fake shot
The share who improved was the same, 20%
20 people whose voices tired easily got a B12 shot and a fake shot in turn. Improvement was similar after both, and 4 of 20 (20%) improved meaningfully either way. The boost people feel from a shot was not coming from B12.
Big trials pooled together: same story
6,276 people and 2,919 people, no difference
An analysis that pooled 16 trials with 6,276 people who were not low in B12 found no gain in memory, focus or mood. In a trial where 2,919 people took B12 and folic acid for 2 years, the decline in physical function and grip strength was no different from a fake pill.
The only energy boost came from a mixed product
32 people, 1.26 times longer running to exhaustion
When 32 people took a product mixing B1, B2, B6 and B12 with taurine and more for 28 days, their running time to exhaustion rose 1.26 times. It was a single study using a product supplied by its maker. Trials of B12 alone changed neither fatigue, mood nor strength.
So who runs low?
Vegans and long-term metformin users
An analysis that pooled 17 studies found that people who eat no meat or dairy at all had lower B12 and more signs of a real shortage in the body. After 5 years on the blood sugar drug metformin, 19.1% had borderline-low levels, about 2 times the 9.5% on a fake pill.
If you are low, pills are enough
Shots and pills restored levels equally
When 153 people low in B12 were split and compared for 3-4 months, oral B12 and shots brought levels back to normal about equally, and pills cost less. Vegans who took a B12 supplement also had better levels on every measure.
Bottom line: B12 is not an energy booster
The formula breaks at the 'more B12 → more energy' link
With normal levels, neither 1,000 µg a day nor a shot reduced fatigue more than a fake. The benefit showed up only when people who were low filled the gap. If you eat no animal foods or take metformin long term, oral B12 does the job.