Several common medications, supplements, and substances can interfere with how your body absorbs or uses vitamin B12. Some block absorption in the gut, others deplete your stores over time, and one can permanently inactivate the vitamin in your bloodstream. Knowing what to watch for helps you avoid a deficiency that can take months or years to develop but causes real neurological and blood-related problems.
Diabetes Medication (Metformin)
Metformin is one of the most widely prescribed drugs in the world, taken by more than 150 million people with type 2 diabetes. It also happens to be one of the biggest culprits behind B12 depletion. Metformin interferes with B12 absorption in the lower part of the small intestine, and because the effect is gradual, many people don’t realize their levels are dropping until symptoms appear.
If you take metformin, periodic B12 blood tests are a reasonable precaution, especially after a few years of use. Supplementing with B12 can offset the loss, but timing matters. Taking B12 at a different time of day than metformin is a simple step, though the core issue is absorption in the gut rather than a direct chemical clash between the two substances.
Acid-Reducing Medications
Proton pump inhibitors (like omeprazole and lansoprazole) and H2 blockers (like famotidine) suppress stomach acid production, which creates a problem for B12 absorption. Your stomach needs acid to separate B12 from the proteins in food. The same cells that produce stomach acid also make intrinsic factor, a compound your body requires to absorb B12 further down in the digestive tract. When these medications dial down acid production, both steps get compromised.
This interaction mainly affects B12 from food sources. Supplemental B12 in pill or sublingual form is already in its free state, so it doesn’t need stomach acid to be released from food proteins. If you’ve been on an acid reducer for more than a year or two, a B12 supplement can help bridge the gap. People on long-term PPI therapy are at meaningfully higher risk of deficiency compared to non-users.
Certain Antibiotics
Chloramphenicol, an antibiotic used for serious bacterial infections, directly interferes with how your body uses B12 to build red blood cells. In patients with B12-related anemia who were given chloramphenicol alongside B12 treatment, the expected recovery in red blood cell production was delayed until after the antibiotic was stopped. When chloramphenicol was introduced after B12 therapy had already started, the initial blood cell response dropped sharply and didn’t recover until about eight days after the last dose of the antibiotic.
This is a temporary effect. Once chloramphenicol clears the system, B12 resumes its normal function. But if you’re being treated for a B12 deficiency and need this antibiotic at the same time, your doctor may need to adjust the timing or monitor your blood counts more closely.
Gout Medication (Colchicine)
Colchicine, commonly prescribed for gout flares, reduces B12 absorption by disrupting the lining of the ileum, the section of the small intestine where B12 is absorbed. Research published in the New England Journal of Medicine found that oral colchicine decreased B12 absorption in nearly all subjects tested, and the effect was strongest when the drug was delivered directly to the ileum.
The good news: this effect appears to be reversible. Colchicine doesn’t damage intrinsic factor or prevent B12 from binding to it. It simply changes how the intestinal wall functions while the drug is active. For people who take colchicine occasionally during gout attacks, this is unlikely to cause a meaningful deficiency. For those on long-term colchicine therapy, monitoring B12 levels is worth discussing.
Potassium Supplements
High-dose potassium chloride, often prescribed alongside certain heart medications, can reduce B12 absorption by making the environment in the ileum more acidic. In a study of 60 patients on potassium chloride therapy, about 30% showed reduced B12 absorption on testing. Patients whose ileal pH dropped to 6 or below absorbed roughly half as much B12 as those with higher pH levels.
That said, the real-world impact tends to be modest. In a separate group of 17 heart patients who took potassium chloride for six months to two years, blood levels of B12 weren’t significantly lower than a control group. Potassium supplementation alone rarely causes outright deficiency, but it can be a contributing factor when combined with other risk factors like age, diet, or additional medications on this list.
Nitrous Oxide
Nitrous oxide, used as an anesthetic during surgery and dental procedures (and sometimes recreationally as “laughing gas”), is uniquely dangerous for B12. Unlike other substances on this list that slow absorption, nitrous oxide permanently destroys the B12 already circulating in your body. It does this by oxidizing the cobalt atom at the center of the B12 molecule, converting it from its active reduced form into inactive forms that your body cannot use.
For most people undergoing a single dental procedure, this isn’t a concern because the body has B12 reserves. But for anyone who is already B12-deficient, even a routine exposure to nitrous oxide can trigger serious neurological symptoms, including numbness, difficulty walking, and cognitive changes. Recreational users who inhale nitrous oxide repeatedly are at particularly high risk. If you know your B12 is low, make sure your dentist or anesthesiologist is aware before any procedure involving nitrous oxide.
Alcohol
Chronic alcohol use impairs B12 status through a less direct but equally effective route: liver damage. Your liver plays a central role in storing and releasing B12, and the widespread inflammation caused by heavy drinking degrades that function over time. B12 deficiency is common among people with alcohol use disorder, and it often shows up as enlarged red blood cells, a condition called macrocytosis, that can be detected on a routine blood test.
A liver function test can help determine whether alcohol-related liver damage is contributing to low B12. Reducing alcohol intake allows the liver to recover some of its storage and release capacity, but supplementation is typically needed in the meantime to correct existing deficiency.
How to Minimize These Interactions
The most practical step is simple awareness. If you take any of the medications above, especially metformin, PPIs, or colchicine on a long-term basis, ask about periodic B12 level checks. Sublingual B12 (dissolved under the tongue) or B12 injections bypass the digestive tract entirely, making them useful options when the problem is gut absorption rather than a direct chemical interaction.
Spacing matters for some interactions. Taking B12 supplements several hours apart from medications like potassium chloride or metformin may reduce competition in the gut, though this won’t fully solve the problem for drugs that change intestinal function over time. For nitrous oxide, the only real protection is making sure your B12 levels are adequate before exposure, since the damage to the vitamin happens in the bloodstream, not the gut.