Can I Take Vitamin B12 with Tresiba (Insulin Degludec)?

Short answer: Yes. Vitamin B12 (cobalamin) has no known pharmacokinetic or pharmacodynamic interaction with Tresiba (insulin degludec), a once-daily ultra-long-acting basal insulin analog approved by the FDA for type 1 and type 2 diabetes in adults and children. The two do not need to be dosed apart, and B12 does not change how degludec is absorbed, metabolized, or how it lowers blood glucose.

The question that actually matters for most people asking this is not whether B12 interacts with Tresiba, but whether another medication in their regimen, usually metformin, is quietly lowering their B12 over time. That is a real and well-documented issue, separate from anything involving Tresiba itself.

Why B12 and Tresiba Don't Interact

Insulin degludec is injected subcutaneously, where it forms soluble multi-hexamer chains that slowly release monomeric insulin into circulation, producing a flat, extended glucose-lowering effect. It binds insulin receptors on muscle, liver, and fat tissue.

Vitamin B12 is a water-soluble vitamin taken orally (or occasionally by injection for deficiency treatment). Dietary and supplemental B12 binds intrinsic factor in the stomach and is absorbed in the terminal ileum via cubilin-mediated uptake, then functions as a cofactor for methionine synthase and methylmalonyl-CoA mutase.

These are unrelated systems: no shared liver enzyme metabolism, no shared transporter, and no evidence that B12 affects insulin receptor binding or that degludec affects intestinal B12 absorption. The FDA-approved Tresiba prescribing information does not list vitamin B12 among interacting substances (label as of its original 2015 approval; confirm the current label version before relying on this for a specific patient, since labels are periodically updated). This is a general safety fact drawn from labeling, not a claim about any individual's other medications.

The Question Underneath the Question: Metformin and B12

Most people who ask about B12 and Tresiba are on Tresiba because they have type 2 diabetes, and most people with type 2 diabetes on insulin are also taking or have taken metformin. Metformin, not Tresiba, is the drug in this picture with a recognized effect on B12 status.

The mechanism generally described in the literature is that metformin interferes with calcium-dependent uptake of the intrinsic factor-B12 complex in the terminal ileum, reducing absorption over months to years of use. A randomized placebo-controlled trial in long-term metformin users (commonly cited as the HOME trial) reported a measurable decline in serum B12 relative to placebo over several years of treatment, and observational studies have found metformin users have meaningfully higher odds of biochemical B12 deficiency than non-users. Exact effect sizes vary across studies and should be verified against the primary papers rather than treated as a fixed number, since reported deficiency prevalence ranges widely (roughly single digits to around one-third of long-term users, depending on dose, duration, and the deficiency threshold used).

Diabetes care guidelines from bodies such as the American Diabetes Association have, in recent editions, supported periodic B12 monitoring in metformin-treated patients, particularly those with anemia or peripheral neuropathy symptoms. The exact wording and recommendation grade change between annual editions of the ADA Standards of Care, so a specific quotation is not reproduced here; check the current year's edition for the precise language.

Why This Overlap Matters Clinically

B12 deficiency and diabetic peripheral neuropathy can look identical: tingling, numbness, and burning in the feet and hands, balance problems, and in more advanced deficiency, cognitive changes. Because both conditions are plausible in the same patient, a clinician who attributes new neuropathy symptoms entirely to diabetes without checking B12 status risks missing a treatable cause. Some studies of metformin-treated patients with B12 deficiency have found worse neuropathy scores than in B12-replete patients, and at least partial reversal with repletion in some patients. This is an area with real evidence but genuine heterogeneity between studies in how neuropathy and deficiency were measured, so it should be read as "plausible and clinically important to rule out," not as a precise, quantified risk for any individual.

Serum B12 alone can be misleading. Methylmalonic acid (MMA) tends to rise before serum B12 falls below a lab's cutoff, and homocysteine can add supporting evidence when elevated alongside MMA. Prolonged, untreated B12 deficiency can cause demyelination that becomes difficult or impossible to fully reverse even after B12 is corrected, this is an established feature of B12 deficiency generally, independent of diabetes, and is a reason not to delay testing when neuropathy symptoms are new or worsening.

Evidence-Status Interaction Assessment

What the Evidence Does Not Establish

There is no clinical trial evidence, to our knowledge, directly testing B12 supplementation timed around Tresiba injections, because there is no proposed mechanism that would make timing relevant. There is also no established universal B12 dose for prevention versus treatment that applies to every patient; published regimens vary, and the right dose and form (cyanocobalamin versus methylcobalamin, oral versus injectable) depends on the severity of deficiency, whether malabsorption is present, and other individual factors that only a clinician reviewing labs can weigh. Anyone with confirmed or suspected B12 deficiency, neurological symptoms, or a history of gastric surgery or pernicious anemia should have dosing individualized rather than following a generic supplement label.

Other Supplements People on Tresiba Commonly Ask About

Vitamin D. Vitamin D deficiency is common in people with type 2 diabetes, and some evidence links low vitamin D to impaired insulin secretion. No interaction with insulin degludec has been described. Whether and how much to supplement should be based on a 25-hydroxyvitamin D level rather than a fixed dose for everyone.

Magnesium. Low magnesium occurs relatively often in type 2 diabetes and may worsen insulin resistance. No known interaction with Tresiba exists; repletion decisions should be based on a measured serum magnesium level.

Alpha-lipoic acid. Alpha-lipoic acid has been studied for diabetic neuropathy symptoms with mixed but generally modest reported benefit in clinical trials. It may have a mild glucose-lowering effect, so anyone adding it to an insulin regimen, including Tresiba, should watch for symptoms of low blood sugar and mention it to their prescriber rather than assuming it is inert.

None of these should be started or stopped based on this article alone if you have kidney disease, are pregnant, or take other medications that affect the same pathways; ask your pharmacist or prescriber to check your full medication list.

If You're Already Taking Both

If you currently take Tresiba and a B12 supplement, there is no evidence-based reason to change that combination. What is worth doing is telling your clinician about the B12 supplement specifically, because supplemental B12 raises serum B12 levels and can mask an underlying functional deficiency unless MMA is also checked. If you take metformin and have never had B12 or MMA measured, ask whether it is due, especially if you have any numbness, tingling, or unexplained fatigue.

When to Seek Care Sooner

Contact your clinician promptly, rather than waiting for a routine visit, if you develop new or worsening numbness, burning, or weakness in your hands or feet, unexplained balance problems, or signs of anemia (fatigue, pale skin, shortness of breath) while on metformin, since these can indicate B12 deficiency that benefits from earlier testing and treatment. This is separate from Tresiba dosing questions, which should also go to your prescriber rather than being adjusted based on supplement use.