Can you overdose on vitamin B12?

Walking down the aisles of any grocery store or health food mart, it’s easy to get overwhelmed by the vast selection of vitamins and supplements. Which are important and what do they even do?

B12, also called cobalamin, is particularly important for everyone to consume as it is not produced by the human body. In fact, it is only produced by microorganisms. Farmed animals obtain their B12 from drinking untreated water, being exposed to bacteria laden manure, or eating fortified feed. For that reason, B12 can be found naturally occurring in animal-based foods, including fish, red meat, dairy products, and eggs. The body naturally stores B12 in the liver, making foie gras a vitamin rich dish.

While most people have a low baseline level of B12, it is especially important for those eating a vegetarian, vegan, or plant-based diet to supplement B12 to ensure healthy levels of this essential vitamin. Nutritional yeast is one commonly fortified product vegans can include in their diet to increase B12 consumption. Those who are pregnant or breastfeeding also require higher levels of B12 to support their growing babies. Older adults who may have increased difficulty absorbing vitamins may also be at risk for a vitamin B12 deficiency.

Consuming enough B12 is only the first step. If there are any problems with absorbing the vitamin, it could also lead to a deficiency. A molecule called intrinsic factor is necessary to make sure the B12 can be transported from the end of the small intestine into the bloodstream. A lack of this intrinsic factor, which can be due to gastric bypass surgery or a disease called pernicious anemia may cause B12 deficiency. Surgically resecting the terminal ileum, a section of the small intestine, (which may be necessary for Crohn disease) can also lead to not having enough B12. Malabsorption can also be caused by drinking too much alcohol, bacterial overgrowth in the gut, inflammation of the small intestine, or celiac disease. Certain drugs (like metformin, used to treat diabetes) can also reduce the absorption of B12.

What does this important vitamin actually do and what happens if you don’t have enough of it? B12 is involved in producing the heme that fills red blood cells, and forming the myelin sheath that covers and protects nerve fibers. When the red blood cells are impacted, they become larger than usual, but do not carry oxygen as efficiently, leading to a form of anemia (called megaloblastic anemia) that can result in feeling fatigued or weak. Impacts on the nervous system may look like having a tingly feeling in your limbs (paresthesias), difficulty speaking or walking (ataxia), or blurry vision (when the optic nerve is affected). Other non-specific symptoms may include depression and constipation.

B12 is part of the vitamins categorized as water-soluble (as opposed to fat soluble), meaning that it gets carried around in the blood dissolved in the water rather than being stored in the cells. Because it is able to easily flow through the blood, this vitamin gets easily filtered by the kidneys and excreted when it is present in excess, making a B12 overdose unlikely.

If you are suffering from any of the above symptoms and suspect you may have a B12 deficiency, it doesn’t hurt to take a supplement as they do not require a prescription. However, it may also be in your best interest to see a healthcare provider for a professional opinion. B12 deficiencies may present similarly to B9 (folate) deficiencies, iron deficiencies, or lead poisoning and severe prolonged B12 deficiency may result in nerve damage, dementia, or even paralysis. To ensure a proper diagnosis, your provider will likely order lab tests including complete blood count (CBC), peripheral blood smear, and reticulocyte count.

Long story short, vitamin B12 is an essential nutrient that plays a critical role in the normal functioning of the nervous system and red blood cells. It is important to ensure that you are getting enough of this vitamin through your diet or supplements, particularly if you are at a higher risk for deficiency. By staying on top of your vitamin B12 intake, you can help maintain your overall health and well-being.