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Vitamin B12 (cyanocobalamin)

Brand names: Nascobal (nasal), B12 injections

Vitamin B12 treats and prevents B12 deficiency, which can cause anemia, tiredness, and nerve problems.

Brand namesNascobal (nasal), B12 injections
Type of medicineVitamin
Prescription?Over the counter (pills); prescription (shots, nasal spray)
ImportantThis page is general information, not a prescription. Don't start, stop, or change a medicine without talking to your provider or pharmacist.

Overview

People with pernicious anemia or after stomach surgery often need shots. Long-term metformin or acid reducers can lower B12.

What it's used for

  • Vitamin B12 deficiency
  • Pernicious anemia
  • Vegan diets

How to take it

  • Pills: take daily with or without food.
  • Shots: follow your schedule, often more often at first.

Your dose depends on your health and other medicines. Always follow your prescription label, and ask your pharmacist if anything is unclear.

Side effects

Common side effects

  • Usually few
  • Pain at the shot site
Call 911Swelling of the face, lips, or throat, hives, or trouble breathing
Call your provider todayLeg cramps, weakness, or irregular heartbeat early in treatment (low potassium)

Warnings

  • Nerve damage from low B12 can become permanent if treatment is delayed.

Other medicines, foods, and alcohol

  • Metformin, PPIs, and H2 blockers can lower B12 absorption over time.

Tell your provider and pharmacist about every medicine, vitamin, and supplement you take.

Pregnancy and breastfeeding

Usual amounts are part of prenatal vitamins.

Missed doses, overdose, and storage

If you miss a dose

Take it as soon as you remember, unless it's almost time for your next dose. Then skip the missed dose. Don't take two doses at once.

If someone takes too much

Call Poison Control at 1-800-222-1222 or get help online at poison.org. Call 911 if the person collapses, has a seizure, has trouble breathing, or can't be woken.

Storing it

Store at room temperature, away from heat and moisture (not in the bathroom), and out of reach of children and pets.

Conditions it's used for

Sources

Last updated 2026-11-13.