Can you take too much vitamin B12?

Why there's no upper limit for B12

The Food and Nutrition Board did not set a Tolerable Upper Intake Level (UL) for vitamin B12 because of its low potential for toxicity. NIH notes that even large doses are generally considered safe.

Adults need 2.4 mcg a day (2.6 mcg in pregnancy and 2.8 mcg while breastfeeding). Supplements can contain far more, such as 500 or 1,000 mcg. You absorb about half of a very small dose (1 to 2 mcg), but much less of a big one.

What a high B12 blood test can mean

MedlinePlus lists a typical serum B12 range of 299 to 1,054 pg/mL; ranges vary by lab. A result above that is uncommon, since extra B12 is usually removed in urine.

• Supplements or shots. These raise blood levels. Tell your clinician what you take and when your last dose or shot was.
• Liver disease, such as cirrhosis or hepatitis.
• Some blood disorders called myeloproliferative disorders, such as polycythemia vera and chronic myelogenous leukemia.

A high result is a reason to ask questions, not a diagnosis. Your clinician can look at it next to your other labs, like a CBC and liver tests. For how the test works and what low results mean, see the vitamin B12 blood test guide or the B12 lab guide. Some medicines affect results; don't stop any medicine before talking to your prescriber.

B12 shots vs pills

B12 shots are prescription medicines, usually given into a muscle. They skip the gut, so they are the usual choice for pernicious anemia and other conditions that block absorption. Some people need monthly shots for the rest of their life, and stopping can let anemia come back and damage nerves.

High-dose pills can work too. NIH cites a 2018 Cochrane review of three trials (153 people) in which 1,000 to 2,000 mcg by mouth raised blood B12 about as well as shots. The evidence was rated low quality. Under-the-tongue tablets work about the same as swallowed ones, and a prescription nasal gel is also available.

Before B12 shots, tell your clinician if you are allergic to cobalt, have Leber's hereditary optic neuropathy (an inherited eye condition) or kidney disease, drink heavily, or are pregnant or breastfeeding.

After a B12 shot, call your clinician right away for hives, rash or itching, difficulty breathing or swallowing, a fast heartbeat, shortness of breath, swelling in the arms or legs, muscle weakness, leg pain or confusion. For severe trouble breathing, call 911.

Who may actually need extra B12

Most people get enough from food: only about 5% of U.S. men and 11% of women fall below the average requirement. B12 is found in fish, meat, poultry, eggs and dairy, and in fortified cereals and nutritional yeast. Groups more likely to run low include:

• Older adults, especially those with atrophic gastritis
• People with pernicious anemia, gut disorders or past stomach or bowel surgery
• Vegetarians and vegans, and infants of mothers who eat a vegan diet
• People who take metformin or long-term acid reducers (PPIs or H2 blockers)

If you're in one of these groups, a B12 test, sometimes with homocysteine or MMA, shows whether you need more.