In this article
Why B12 and energy get talked about together
You're tired in a way sleep doesn't fix. A friend swears by her B12 shot. It's a fair thing to wonder about, because the link between B12 and energy is real. It just isn't the link most ads suggest.
Your body needs vitamin B12 to make healthy red blood cells and to keep nerves working. When B12 runs low for long enough, you can develop a type of anemia. MedlinePlus lists fatigue, lack of energy, or lightheadedness when standing up or exerting yourself among its common symptoms [1]. Some people with low B12 have no symptoms at all, or only mild ones [1].
Left untreated, low B12 can also affect your nerves and brain. The National Heart, Lung, and Blood Institute (NHLBI) lists tingling or pain, trouble walking, confusion, memory loss, mood changes, vision problems, and a sore, smooth, red tongue among the later symptoms [2]. MedlinePlus notes that low B12 over a long time can cause nerve damage [1].
So yes, correcting a B12 deficiency can lift the tiredness that the deficiency caused. The open question is what happens when your B12 is already fine. We'll get to that.
Who is most likely to run low on B12
B12 deficiency is common, and it becomes more common with age [5]. NHLBI explains that older adults are more likely to have digestive problems that make it harder to absorb B12 from food [2]. Here are the groups the research points to.
| Risk | What the research found |
|---|---|
| Getting older | Deficiency becomes more common with age [5], partly because absorption from food gets harder [2]. |
| Long-term metformin | In a diabetes-prevention study, low or borderline-low B12 was more common on metformin after 5 years (19.1% vs 9.5% on placebo) and after 13 years (20.3% vs 15.6%) [3]. |
| Long-term acid reducers | In a study of more than 200,000 people, 2 or more years of a proton pump inhibitor was linked to higher odds of B12 deficiency, and so were H2 blockers, to a lesser degree [4]. |
| Vegan diet | A diet with no animal products, including no milk or eggs, supplies no B12, so regular B12 supplements are advised [6]. |
| Pernicious anemia | An autoimmune condition in which the body destroys the stomach cells that make intrinsic factor, the protein needed to absorb B12 [1]. |
| Stomach or bowel surgery | Surgery that removes parts of the stomach or small intestine, including some weight-loss operations, can cut absorption [1]. |
Metformin
Metformin is a common diabetes medicine, and many women take it in midlife. The Diabetes Prevention Program followed more than 2,000 adults with high blood sugar and overweight. People assigned to metformin, 850 mg twice a day, were more likely to have low or borderline-low B12 than people on placebo, and each extra year on metformin raised the odds of deficiency by about 13% [3]. Nerve problems were more common in metformin users who had low B12 [3]. The authors suggested that doctors consider checking B12 regularly in people on metformin [3].
Acid reducers
Stomach acid helps release B12 from food. A Kaiser Permanente study compared 25,956 people diagnosed with B12 deficiency with 184,199 people without it [4]. Having filled 2 or more years of a proton pump inhibitor (such as omeprazole) was linked to 65% higher odds of deficiency, and 2 or more years of an H2 blocker to 25% higher odds [4]. Higher daily doses of proton pump inhibitors showed a stronger link [4]. This kind of study shows an association, not proof that the medicine caused the deficiency.
Vegan and strict vegetarian diets
B12 comes from animal foods. The FDA-approved label for cyanocobalamin injection states that a diet with no animal products, including no milk products or eggs, does not supply any B12, and that people eating this way should take oral B12 regularly [6]. NHLBI notes that diet alone is a rare cause of deficiency in the US [2].
Pernicious anemia and other autoimmune conditions
In pernicious anemia, your body can't make enough intrinsic factor, so B12 from food can't be absorbed well [1]. NHLBI lists other autoimmune conditions, such as celiac disease, type 1 diabetes, and thyroid disease, among the conditions that raise risk, and notes that some people need lifelong treatment [2].
B12 shots vs high-dose pills
Many people assume a shot is always stronger than a pill. For a lot of people with low B12, the evidence doesn't back that up.
A Cochrane review looked for randomized trials comparing oral B12 with B12 injections into the muscle in people with a deficiency [5]. It found three trials with 153 people in total, aged on average from about 39 to 72, treated for three to four months [5]. In two trials, 1,000 mcg of oral B12 a day raised blood B12 about as well as injections. In the third, 2,000 mcg a day by mouth raised levels more than injections did [5].
The reviewers rated this evidence as low quality, mainly because the trials were small, and none of them measured symptoms like tiredness or quality of life [5]. In one trial, oral treatment cost less [5]. MedlinePlus says the same in plain terms: some people can be treated well with large doses of B12 by mouth, while others need a shot, often once a month [1].
Pernicious anemia is the big exception. The FDA-approved cyanocobalamin injection label calls injected B12 the recommended treatment for pernicious anemia, needed for life, and says the oral form is not dependable for it [6]. For people who absorb B12 normally, the same label says long-term treatment should be with an oral B12 product [6]. The Cochrane trials and the label don't fully agree on pills in pernicious anemia, which is one more reason to decide this with your clinician rather than on your own.
Shots also make sense when a clinician wants to correct a deficiency quickly, or when pills haven't worked. NHLBI notes that for a serious deficiency, a doctor may recommend shots until levels are healthy [2].
Will a B12 shot give you energy if your levels are normal?
This is the question most people are really asking, and the honest answer is that there's no good evidence it will.
A 2021 systematic review pooled 16 randomized trials with 6,276 people who did not have advanced nerve disease, testing B12 alone or with other B vitamins [7]. B12 did not improve memory, thinking, or depressive symptoms [7]. The review also set out to look at fatigue, but only one of the 16 trials measured it, so there wasn't enough data to draw any conclusion about energy [7].
That's not the same as proof that B12 shots never help anyone's energy. It means nobody has run the trials that would show it, in people whose B12 is already normal. If someone promises a boost from extra B12 alone, they're going beyond what has been tested.
There's also a practical point. Your body doesn't hold on to much of an injected dose it doesn't need. According to the cyanocobalamin injection label, within 48 hours after an injection of 100 or 1,000 mcg, 50% to 98% of the dose may show up in the urine, most of it in the first eight hours [6].
Tiredness has many causes, including poor sleep, low iron, thyroid problems, depression, perimenopause, and medicines. If you've been tired for weeks, ask your clinician to look for the cause rather than treating it with a vitamin first.
Side effects and safety
B12 injections are generally well tolerated, but they're a real medicine with real warnings. The FDA-approved cyanocobalamin injection label lists:
- Mild, short-lived diarrhea, itching, and a passing rash [6]
- A feeling of swelling of the whole body [6]
- Rare but serious allergic reactions; anaphylactic shock and death have been reported after injected B12 [6]
- Severe, fast vision loss in people with early Leber's disease, an inherited optic nerve condition [6]
- Low potassium and sudden death when severe anemia is treated intensively, which is why potassium is watched closely early in treatment for pernicious anemia [6]
The label also says a person with a sensitivity to cobalt or B12 should not use it [6]. Tell your clinician about any allergies and any eye conditions before you start.
How WarriorBabe Health's B12 injection fits in
Our B12 injection is methylcobalamin, a form of B12, at 5 mg/mL in a 10 mL vial, injected into the muscle once a week at the dose your provider prescribes. It's meant for women with low or borderline B12. Our MIC + B12 injection combines B12 with other nutrients; you can read what's known about those in our guide to lipotropic injections.
Both are made by a licensed compounding pharmacy for you, under your provider's prescription. Compounded drugs are not FDA-approved, which means the FDA does not verify their safety, effectiveness, or quality before they are sold [8]. The label facts above come from an FDA-approved cyanocobalamin product, a different form of B12 from the methylcobalamin we use.
A sensible order of steps: check your B12 with a blood test, look at the reasons yours might be low (medicines, diet, digestion, age), and then choose shots or pills with your clinician. If your level is normal, a shot is unlikely to be the fix, and the search for the cause of your tiredness should go on. You can see all our options on the energy treatments page.
Frequently asked questions
How fast do B12 shots work for energy?
We can't give you a timeline. The trials comparing shots and pills measured blood B12 levels over three to four months, not how quickly people felt better [5]. If you have a true deficiency, ask your clinician how and when they'll recheck you.
Are B12 pills as good as shots?
For many people with low B12, high-dose pills (1,000 to 2,000 mcg a day) raised blood levels about as well as shots in small trials, though the evidence is low quality [5]. For pernicious anemia, the FDA-approved injection label calls injections the recommended treatment and says the oral form is not dependable [6].
Should I get my B12 checked if I take metformin?
It's worth asking. In a long-term study, metformin users were more likely to have low or borderline-low B12, and the researchers suggested routine testing be considered [3].
Can a B12 shot help if my levels are normal?
There's no good evidence that it does. A review of 16 trials found B12 didn't improve memory or mood, and only one trial measured fatigue, which wasn't enough to answer the question [7].
Is a B12 injection safe?
It's generally well tolerated, but serious allergic reactions have been reported, and people with early Leber's disease or a cobalt or B12 sensitivity should not use it [6]. Tell your clinician about your allergies and medical history first.
References
- MedlinePlus Medical Encyclopedia. Vitamin B12 deficiency anemia. US National Library of Medicine. medlineplus.gov/ency/article/000574.htm
- National Heart, Lung, and Blood Institute. Anemia: vitamin B12-deficiency anemia. National Institutes of Health. Updated March 24, 2022. www.nhlbi.nih.gov/health/anemia/vitamin-b12-deficiency-anemia
- Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016;101(4):1754-1761. pubmed.ncbi.nlm.nih.gov/26900641/
- Lam JR, Schneider JL, Zhao W, Corley DA. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. 2013;310(22):2435-2442. pubmed.ncbi.nlm.nih.gov/24327038/
- Wang H, Li L, Qin LL, et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev. 2018;3(3):CD004655. pubmed.ncbi.nlm.nih.gov/29543316/
- Cyanocobalamin Injection, USP 1000 mcg/mL. Prescribing information (Henry Schein). DailyMed, US National Library of Medicine. Revised 02/2024. dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=92f08572-cfd9-4a99-bb49-937ab94eeb62
- Markun S, Gravestock I, Jäger L, et al. Effects of vitamin B12 supplementation on cognitive function, depressive symptoms, and fatigue: a systematic review, meta-analysis, and meta-regression. Nutrients. 2021;13(3):923. pubmed.ncbi.nlm.nih.gov/33809274/
- US Food and Drug Administration. Compounding and the FDA: questions and answers. Content current as of September 16, 2025. www.fda.gov/drugs/human-drug-compounding/compounding-and-fda-questions-and-answers
This article is for general information. It is not medical advice and does not replace a visit with a licensed clinician who knows your history. Do not start, stop or change a medication without talking to your clinician. If you think you are having a medical emergency, call 911.
