B12 (cobalamin) is essential for four critical processes: red blood cell production (low B12 causes megaloblastic anemia), myelin sheath maintenance on nerves (deficiency causes neuropathy and numbness), DNA synthesis in every dividing cell, and one-carbon metabolism — the recycling of homocysteine into methionine, which supports brain chemistry and cardiovascular health. B12 is only naturally found in animal products, and absorption depends on intrinsic factor produced in the stomach.

Injections are especially helpful for patients with impaired absorption or elevated demand: people taking proton pump inhibitors (PPIs like omeprazole) or H2 blockers, long-term metformin users, vegetarians and vegans, patients with pernicious anemia, Crohn’s or celiac disease, post-bariatric surgery patients, and adults over 50 whose stomach acid has declined. Anyone with persistent fatigue, brain fog, tingling in the hands or feet, or unexplained mood changes should have their B12 level checked.

Oral B12 absorption depends on stomach acid, intrinsic factor, and a healthy small intestine — and even in healthy adults, only about 1–2% of an oral dose is absorbed. If you have any of the conditions above, that number drops further or fails entirely. Subcutaneous or intramuscular injection bypasses the gut and delivers B12 directly into circulation, raising serum levels reliably regardless of absorption issues. If oral supplements haven’t worked for you, injections usually do.

Methylcobalamin is the active, bioavailable form your body uses directly in one-carbon metabolism and nerve support. Cyanocobalamin is a synthetic precursor your body must convert into active forms — it’s cheaper and extremely stable, which is why it’s common in generic supplements. Both raise serum B12 effectively, but many providers (and patients with genetic methylation variants like MTHFR) prefer methylcobalamin for its direct action. Your provider will recommend what fits your health profile.

Most patients inject once weekly for the first 4–8 weeks to restore serum levels, then shift to maintenance dosing of once weekly or every other week. Your provider can adjust based on symptoms and, if appropriate, follow-up labs. Injections are small, the needle is tiny, and most women do it in under 30 seconds at home using the supplies included in every shipment.

Do not use if you have a known hypersensitivity to cobalamin, cobalt, or any excipient. B12 is contraindicated in patients with Leber’s hereditary optic neuropathy, a rare genetic eye condition — high-dose B12 can worsen optic nerve damage. Patients with polycythemia vera or active cancers affecting red blood cell lines should discuss with their provider. B12 is generally safe during pregnancy and breastfeeding at recommended doses, but always confirm with your provider during intake.