Most people have healthy levels of vitamin B12. But some don’t. What you do or don’t eat, along with certain health conditions and medications, could be the reason why. When left untreated, a B12 deficiency can lead to problems with your heart, nerves, or brain.

B12 shots, supplements, and changing what you eat can help. But first, the doctor has to find out why you’re not getting enough. Here are some of the most common causes they’ll look for.

Pernicious Anemia

This disorder causes immune system to attack cells in the stomach that make intrinsic factor (IF). That’s a protein you need to absorb B12. Experts aren’t sure what causes pernicious anemia. Some people are born with it, but that’s rare.

Symptoms can include:

Tiredness
Pale skin
Weakness
Fast heart rate

Your body also needs B12 to make red blood cells. That’s what carries oxygen to your tissue and organs. You can become anemic when you don’t have enough.

“Pernicious” means deadly. But it’s not so deadly anymore. Today, there are simple ways to treat this kind of anemia. Your doctor will likely give you a B12 shot in your muscle. You’ll get it once a week until your levels are normal. After that, you may need a shot or high-dose supplement once a month for the rest of your life. If you don’t like shots or pills, ask your doctor if a B12 nasal spray is right for you.

Age

Some older adults have a condition called atrophic gastritis. That’s when your stomach doesn’t make enough hydrochloric acid. It helps you absorb B12 from food.

It’s usually easier to absorb B12 from supplements or fortified foods when you have atrophic gastritis. That’s why it’s suggested that people 50 and older get their B12 from these sources. But sometimes older adults need higher dose to keep their levels up. Ask your doctor what’s right for you.

Intestinal Disorders

Certain health conditions make it hard for your small intestine to absorb B12. They include:

Inflammatory bowel disease (IBD), such as Crohn’s or ulcerative colitis
Celiac disease

Symptoms can include:

Trouble thinking clearly
Megaloblastic anemia, when your red blood cells are bigger than normal and there’s not enough of them
Dementia

Treatment includes managing your IBD or celiac disease. But you may need extra B12 through shots or supplements, too.

Medication

Certain drugs may lower your B12 levels, especially if you use them for a long time. These include medications for:

Heartburn: These drugs may block the stomach acid that helps you absorb B12. They include proton pump inhibitors and H2 receptor agonists. These are used to treat acid reflux and peptic ulcer disease.

Diabetes: Experts think long-term use of metformin can lead to malabsorption issues.

Seizures: There’s growing evidence that antiepileptic drugs may affect how you absorb B12.

Infections. An antibiotic called chloramphenicol may cause problems with B12 absorption.

If you take these medicines every day, your doctor may want to check your B12 levels every year or so. They may change your prescription or recommend supplements if your levels get too low.

Stomach Surgery

Your body can’t absorb B12 as well if all or part of your stomach or small intestine is taken out. That can happen if you have surgery for weight loss, cancer, or other health conditions.

If you have surgery on your gastrointestinal tract, your doctor will want to keep an eye on your B12 levels. They can give you shots of B12 if they get too low.

Vegan or Vegetarian Diet

Most natural sources of B12 come from animal products. That includes meat, dairy, and eggs. But it’s possible to get enough B12 on a vegan or strict vegetarian diet. The easiest way to do that is to take a daily or weekly supplement. Ask your doctor what dose is right for you.

Here are some other plant-based sources of B12:

Fortified nondairy milks
Fortified cereals
Vitamin waters or energy drinks
Tempeh

Getting enough B12 is really important if you’re pregnant or breastfeeding. That’s because a deficiency could harm you and your baby. Ask your doctor what kind of supplements you or your child need.