Fast Facts:

— B12 is a nutrient needed to form red blood cells, DNA, and brain and nerve cells.

— In deficient people, the benefits of B12 shots include improved mood and increased energy levels.

— If a doctor prescribes B12 injections, symptoms like fatigue, weakness, depression, and more may resolve within generally resolve in six to 12 weeks.

B12 has become something of an “it supplement” lately, with celebrities getting B12 shots after pulling all-nighters and athletes using them to enhance recovery. But in terms of the science, what are the benefits of vitamin B12 injections?

A study in the journal Mayo Clinic Proceedings (1) sheds light on some of the potential benefits of B12 injections for people with a deficiency. Researchers shared that one 55-year-old woman who struggled with depression and symptoms such as numbness in her feet saw her symptoms gradually lessen within three to four weeks of getting B12 shots. A 17-year-old saw symptoms such as fatigue, dizziness, and nausea she’d battled for two years either reduce considerably—some even vanished. A 62-year-old woman who struggled with tiredness and trouble concentrating after gastric bypass surgery saw her symptoms resolve almost entirely within two months after starting B12 shots.

These quick boosts of B12 have been touted as everything from a weight-loss tool to an energy booster to a workout recovery aid. But if you’ve been feeling run down, can B12 shots help you? Keep reading to find out all the details.

What is B12?

Also known as cobalamin, B12 is a micronutrient your body needs to form DNA (2) and red blood cells.. It helps with nerve function, DNA repair, and red blood cell health.

How Can You Take B12?

Since our bodies don’t make B12, you have to consume it through food. Animal products like meat, eggs, fish, and dairy contain B12; so do some fortified foods, like certain cereals. But you can also dose up through shots or supplements.

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How do B12 Shots Work?

B12 can be injected into muscle, usually the arm or buttocks. The injections sit in the tissue as kind of a reservoir and slowly leak out over time into the blood.

After an initial round of closely spaced injections, patients can often go for a shot just once a month, instead of taking a daily supplement.

B12 Shots vs Pills

There are lots of over-the-counter forms of B12 to choose from, including pills, lozenges, and sublingual tablets. There’s also a prescription B12 nasal spray.

If you opt for oral supplements, your body absorbs the B12 in your G.I. tract—similar to how it’s processed when food is your source. This uptake depends on a protein called intrinsic factor (IF), which captures B12 in the small intestine, absorbing it in your small intestine.

If you have a healthy digestive system, you’ll very efficiently absorb the first two micrograms of an oral dose (about 70 to 90 percent of it); after that, you’ll take in a small additional amount as it diffuses across the mucus membranes of your G.I. tract. However, if you lack IF (the case for people with pernicious anemia; see below), your small intestine can’t absorb supplemental or dietary B12, so you’ll only glean a small fraction (about 1 to 2 percent) of the dose.

With sublingual or intranasal B12, the vitamin seeps through the linings of your mouth, nose, or G.I. tract. That means you’ll only capture 1 to 2 percent of the dose (as mentioned above), making this an inefficient way to increase your B12 levels.

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5 Health Benefits of B12 Injections

1. More Energy

If you’re fatigued due to deficiency, B12 shots may help recharge your battery. Fatigue may be a problem for those in particular with pernicious anemia, a cause of low B12. Pernicious anemia develops when your body cannot make enough healthy red blood cells because it doesn’t have enough vitamin B12. This can lead to low energy levels. See below for more on whether non-deficient folks should consider B12 shots to improve their energy.

2. Improved Metabolism and Weight Loss

Many people suffer from a slow metabolism, causing weight gain and fatigue. With a B12 injection, you can boost your metabolism and speed up the body’s processes.

A higher metabolism will help you burn calories faster while converting food into energy far more efficiently. There is a growing body of evidence suggesting that people with lower blood levels of vitamin B12 are more likely to be obese.

3. Better Mood

Evidence has linked depression with B12 deficiency. So if you’re short on the nutrient, the shots may help normalize your mood. Of course, if you’re depressed, you shouldn’t self-medicate with any supplement, B12 included. Make seeing your healthcare professional a priority, and while you work toward a diagnosis, you can ask your provider to prescribe B12 injections.

Keep in mind: You may not see improvement if B12 deficiency isn’t the underlying cause of your depression. In a study (3) of people with normal levels of the vitamin, a B12 supplement didn’t have an effect on depression.

4. Bone Health

If your body lacks B12, your skeleton may suffer: in a study published in the Journal of Bone and Mineral Research (4), low B12 was linked with poor bone mineral density, suggesting a role for the vitamin in bone formation. Those who are deficient may also be at higher risk of osteoporosis and hip fractures, a known complication of pernicious anemia.

5. Improved Memory

Normal B12 status is important in the preservation of normal cognitive function. In fact, people with vitamin B12 levels on the lower side of normal performed worse on memory tests than those with high-normal levels in a study in The American Journal of Clinical Nutrition (5). Even after age differences were accounted for, the effect on memory remained. Why? Inadequate B12 has been associated with degeneration of neurons and atrophy of your brain’s gray matter, which is critical for information processing.

Who Should Get B12 Injections?

People who will get the biggest benefits of B12 injections fall into these categories:

1) People with B12 deficiency due to malabsorption

2) People who don’t get enough B12 in their diet

Vitamin B12 deficiency is notoriously tough to diagnose—the blood tests are prone to both false positives and false negatives (7)—and doctors don’t always agree on which combination of tests to use. Yet, B12 deficiency is clearly a problem: In the United States, it is estimated that roughly 6 percent of people under age 60 (8) are short on the micronutrient. That number rises to 1 in 5 in the over-60 population. There are also lots of people who land in the gray area, where they’re not clinically deficient, but don’t have adequate levels, either.

One of the most common culprits behind B12 deficiency is malabsorption, where a step in the breakdown and absorption of B12 goes awry, preventing your system from soaking it up. In these cases, even the largest oral dose won’t significantly spike your levels, since your body is unable to utilize the vitamin. That makes injections your best bet if you’re dealing with conditions such as pernicious anemia, atrophic gastritis, bariatric surgery, taking certain medications such as Nexium, Prilosec, Zantac, Pepsid, and Metformin, or have autoimmune digestive conditions such as Celiac or Chron’s.

Non-meat-eaters don’t have a problem absorbing B12—they just don’t consume enough of it. That means vegans and vegetarians may respond well to B12 supplements or injections.

Can I Get B12 Injections If I’m Not Deficient?

Non-deficient people report a lift in energy after taking B12 injections, but there’s not much scientific evidence that this actually works.

What we do know: In one Dutch study when people with IBS or IBD and normal B12 levels were given a daily B12 supplement, they didn’t report any significant improvements in fatigue, depression, or quality of life. There was, however, a noticeable spike in “motivation” in the B12 group.

Similarly, in a Swiss study (11), almost half of all general practitioners agreed with prescribing B12 for unexplained fatigue.

B12 Injection Side Effects

You don’t have to worry about overdoing it: There’s no established toxic dose for B12—since it’s a water-soluble vitamin, you’ll flush out whatever you can’t use. The main complaint is mild to moderate pain, particularly in thin people (6). There might be some discomfort from the injection, not much different from getting a vaccination.

Other common side effects include diarrhea, headache, and swelling (and not only at the site of the injection). If you experience any uncommon side effects, such as chest pain or hives, contact your health care provider immediately.