Several common medications, medical conditions, and lifestyle factors can block your body from absorbing vitamin B12. Because your liver stores large reserves of B12, a new absorption problem can take 5 to 10 years to show up as a clinical deficiency, which means many people have no idea anything is wrong until symptoms appear.
To understand what goes wrong, it helps to know how the process normally works. B12 absorption is a multi-step chain, and a break at any point can shut it down.
How B12 Absorption Works
When you eat foods containing B12, your stomach acid first separates the vitamin from the protein it’s bound to. A carrier protein in your saliva and stomach grabs the freed B12 and escorts it into the small intestine. There, pancreatic enzymes release B12 from that carrier, and it attaches to a second protein called intrinsic factor, which is produced by cells in the stomach lining. This intrinsic factor-B12 complex then travels to the very end of the small intestine, a section called the terminal ileum, where specialized receptors pull it into the bloodstream.
Every link in this chain matters: stomach acid, intrinsic factor, a healthy terminal ileum. Knock out any one of them and B12 absorption drops significantly or stops entirely.
Medications That Interfere With B12
Acid-Suppressing Drugs
Proton pump inhibitors (PPIs) like omeprazole and lansoprazole are among the most widely used medications in the world. They work by drastically reducing stomach acid production, which is exactly the first step B12 needs. Without enough acid, B12 stays locked to the food proteins it arrived with and passes through the gut unabsorbed. Long-term PPI use also promotes bacterial overgrowth in the digestive tract, which can further reduce B12 availability. H2 blockers (like famotidine) have a similar but generally milder effect.
Short courses of these medications are unlikely to cause problems. The risk climbs with use lasting a year or more, especially at higher doses.
Metformin
Metformin, the most common medication for type 2 diabetes, blocks B12 absorption through what appears to be interference with a calcium-dependent step. In the terminal ileum, the intrinsic factor-B12 complex needs calcium to bind to the receptor that pulls B12 into your cells. Metformin disrupts that binding. Multiple observational studies and meta-analyses have confirmed a significant link between metformin use and B12 deficiency, and the risk increases with higher doses and longer duration.
Autoimmune Destruction: Pernicious Anemia
Pernicious anemia is an autoimmune condition where the immune system produces antibodies that attack intrinsic factor. Without functional intrinsic factor, B12 has no way to latch onto the receptors in the terminal ileum. It simply passes through and is excreted. This is one of the most complete blocks to B12 absorption because it eliminates an essential step entirely. Pernicious anemia is diagnosed through a blood test that detects anti-intrinsic factor antibodies, and people with this condition typically need B12 injections because oral supplements also depend on intrinsic factor to be absorbed.
Gastrointestinal Conditions
Crohn’s Disease
Crohn’s disease causes chronic inflammation anywhere in the digestive tract, but the area it most commonly affects is the terminal ileum, the exact stretch of intestine responsible for absorbing B12. When that tissue is inflamed, scarred, or surgically removed, the receptors needed to take in B12 are damaged or gone. People with Crohn’s involving the ileum are at particularly high risk and often need monitoring even during remission.
Celiac Disease
Celiac disease damages the lining of the small intestine through an immune reaction to gluten. While the damage primarily affects the upper small intestine, it can impair overall nutrient absorption enough to contribute to low B12 levels, especially when the condition goes undiagnosed for years.
Gastritis
Chronic inflammation of the stomach lining, whether caused by infection (commonly H. pylori), autoimmune processes, or long-term irritation, reduces hydrochloric acid production. Since stomach acid is the first thing needed to free B12 from food, chronic gastritis is one of the more common causes of B12 deficiency, particularly in older adults whose stomach lining has thinned with age.
Surgical Causes
Weight loss surgeries, particularly Roux-en-Y gastric bypass, physically rearrange the digestive tract. They reduce the size of the stomach (cutting intrinsic factor production) and reroute food past sections of the intestine (reducing absorption surface). In one long-term study following patients eight years after bariatric surgery, 80.7% had at least one nutritional deficiency despite good adherence to prescribed supplements. B12 deficiency rates were higher than in the general population across all procedure types.
Any surgery that removes part of the stomach or the terminal ileum, whether for weight loss, cancer, or Crohn’s disease, can permanently impair B12 absorption. These patients typically require lifelong B12 supplementation, often by injection.
Alcohol and B12
Chronic heavy drinking damages the digestive system at multiple points. Alcohol irritates and inflames the stomach lining, reducing acid production. It can also impair pancreatic function, which is needed to transfer B12 to intrinsic factor in the small intestine. The Cleveland Clinic lists alcohol use disorder as a direct cause of B12 deficiency because of this cumulative digestive damage. Reducing or eliminating alcohol can allow the gut to heal and restore some absorptive capacity, though long-standing damage may not fully reverse.
Nitrous Oxide Destroys Active B12
Nitrous oxide (laughing gas) doesn’t block absorption in the traditional sense. Instead, it destroys B12 that’s already in your body. The gas chemically reacts with the cobalt atom at the center of the B12 molecule, permanently inactivating it through a process that generates highly reactive oxidants and damages the enzymes that depend on B12. A single prolonged dental or surgical exposure is enough to cause problems in someone whose B12 is already borderline. Recreational use is an increasingly recognized cause of severe B12 deficiency, sometimes leading to nerve damage that can take months to recover from, if it recovers at all.
Age-Related Decline
Even without a specific disease or medication, B12 absorption naturally decreases with age. Older adults produce less stomach acid and less intrinsic factor. Estimates suggest that 6% or more of adults under 60 have B12 deficiency, with rates climbing higher in people over 60. This is why many health guidelines recommend that older adults get their B12 from supplements or fortified foods rather than relying solely on meat and dairy, since the crystalline form in supplements doesn’t need stomach acid to be freed from food proteins.
Why Deficiency Takes Years to Appear
Your liver is an efficient B12 warehouse. It stores enough to last years even if absorption stops completely. This means a new absorption problem, whether from starting metformin, developing pernicious anemia, or undergoing surgery, can take 5 to 10 years to produce noticeable symptoms. By the time fatigue, numbness, tingling, or cognitive changes appear, stores are deeply depleted. This long lag is why people with known risk factors benefit from periodic blood testing rather than waiting for symptoms.