Feeling low-energy? You could be among the estimated 3.6% of Americans who are deficient and 12.5% who get insufficient amounts of vitamin B12.

The first step to finding out is seeing your doctor, as Dr. Elizabeth Sharp, internist and medical director at Health Meets Wellness in New York City, previously told Fortune.

“If someone is dealing with low energy, my initial response would be to recommend a series of tests before considering supplements,” she said. Testing for vitamin deficiencies, including vitamin B12, she says, is “crucial,” especially in vegans and vegetarians.

What does vitamin B12 do?

Vitamin B12, according to the National Institutes of Health Office of Dietary Supplements, is a nutrient (or family of molecules called the cobalmins that incorporate an ion of the element cobalt in their structure, according to Discover) that helps keep your body’s blood and nerve cells healthy and helps make DNA, the genetic material in your cells. It also helps prevent megaloblastic anemia, a blood condition that may be the culprit behind someone feeling tired and weak.

Vitamin B12 deficiency symptoms

In addition to weakness and fatigue, a B12 deficiency may also prompt a slew of other symptoms, depending on the severity, including:

• pale skin

• heart palpitations

• loss of appetite

• weight loss

• infertility

• tingly hands and feet due to nerve problems

• balance issues

• depression

• confusion

• dementia

• poor memory

• mouth soreness

How much vitamin B12 do I need per day?

Everyone requires a certain amount of B12 depending on their age, starting with .5 mcg per day for infants up to 2.4 mcg for teens and adults and even more—2.6 mcg and 2.8 mcg, respectively, for women who are pregnant or breastfeeding.

Vitamin B12 foods

B12 is found in a wide array of animal-based foods, as it’s produced by a gut bacteria that is then accumulated in the muscle tissue of most herbivorous animals, including fish, cows, chickens, and some shellfish, throughout their lives; additionally, many livestock are supplemented with B12 through their feed. When it’s absorbed by the animal, it is in turn passed on to people who eat animal-based products such as dairy and meat.

Vitamin B12 does also accumulate in a few plant-based sources, including some mushrooms, seaweed, and nutritional yeast, but not enough to fortify vegans, who, in particular, should consider being tested for deficiency and then boosting intake with both B12-fortified foods, such as cereals and plant-based milks, and supplements.

Some other populations are also more likely to be deficient, according to Cathi Dennehy, a doctor of pharmacy, dietary supplement researcher, and professor at the University of California San Francisco, who previously spoke with Fortune about the vitamin. That includes folks with an autoimmune disorder called pernicious anemia, who are unable to absorb B12, and people who take a lot of over-the-counter stomach-acid reducers, like famotidine, because they don’t have the stomach acid to release the B12 from the animal protein.

B12 supplements for energy

Taking vitamin supplements is a simple way to boost your B12 intake. But be selective when making your purchase.

“Be wary of supplements that are far cheaper than the rest or look like a good deal,” Jolene Brighten, naturopathic endocrinologist and author of Is This Normal?, told Fortune previously. “Often these are using poor quality ingredients and in some cases, because they are not regulated, they won’t actually have what the label lists.”

To help avoid such risks, suggests Dennehy, start by speaking with your health care provider or a registered dietitian who is well-versed in supplements. Then, when you’re ready to buy, be sure to do some research, especially on the brand.

Consumers may want to consult the NIH Office of Dietary Supplements as well as subscription-based sources, such as Consumer Lab, which charges a fee for access to its test results, which looks into issues ranging from possible contaminants to making sure a supplement contains what it claims to, Dennehy previously told Fortune, noting, “A question that I get asked fairly frequently is, ‘Is this brand that I’m taking a good brand?’”

Vitamin B12 can be found in multivitamins, B-complex supplements, and by itself in a B12 supplement, which are available in many natural and synthetic forms including cyanocobalamin and adenosylcobalamin—some in a sublingual pill that dissolves under the tongue. But research has not shown that any form of supplemental vitamin B12 is better than others, according to the NIH Office of Dietary Supplements. Further, notes the NIH, some come in doses much higher than recommended amounts, such as 500 mcg or 1,000 mcg, but because your body absorbs only a small percentage, those doses are considered safe.

Other ways to get extra B12

A prescription form of vitamin B12 can be given as an injection, notes the NIH—typically for people with deficiencies who also have problems absorbing vitamin B12 as well as those who have undergone gastric surgery, as shots enable the body to absorb vitamin B12 without going through the digestive system, according to Medical News Today.

Vitamin B12 is also available by prescription as a nasal gel that’s sprayed into the nose—as well as through IV infusions, although there are mixed opinions on the efficacy and safety of this delivery method.

But finally, stressed Dennehy, however you opt to supplement yourself, “the only reason it would improve energy would be if you’re correcting a deficiency.”