How much B12 should you take per day?

B12 is essential for maintaining the normal function of the brain and nervous system. It is required for healthy red blood cell production, maintenance of the myelin sheath, synthesis of DNA, and metabolism of homocysteine.

B12 also works with other B vitamins in converting the food you eat into energy. It acts as a cofactor for methionine synthase, which is required for the production of SAMe, the methyl donor involved in synthesis of serotonin and dopamine.

Vitamin B12 is available in four different forms. The three natural, bioactive forms include methylcobalamin, adenosylcobalamin, and hydroxocobalamin. The synthetic form, cyanocobalamin, is the form most commonly used in supplements and foods, as it is cheap to produce. Approximately 50-98% of the injected cyanocobalamin is in the urine, mostly excreted within the first eight hours.¹

• Methylcobalamin occurs naturally in the body and assists in producing red blood cells, DNA and other physiologic functions. 
• Adenosylcobalamin also exists naturally, is required for energy metabolism, but is less commonly used in vitamin B12 supplements. Adenosylcobalamin is the active form used by the mitochondria (the powerhouse of every cell).²
• Hydroxocobalamin is a precursor to methylcobalamin and adenosylcobalamin. It is most commonly used in injections and is retained in the body for longer. 
• Cyanocobalamin must be converted to the active form before it can be used in the body. However, this conversion process may be impaired in people with MTR/MTRR mutations.

Many factors affect the way B12 is absorbed and used in the body, including gut health, age, diet, interference with medications and health conditions.

Certain genetic variants may affect the proteins involved in vitamin B12 absorption, cellular uptake and intracellular metabolism. For example, mutations of the MTR or MTRR genes can affect the conversion of B12 to its active form, which affects the absorption of synthetic cobalamin in the body.

Vitamin B12 dosage recommendations as per guidelines from WHO and NIH.³

The absorption rate of a 500 mcg B12 dose is only around 2%, while a 1000mcg dose is around 1.3%.¹⁴

Adults under 50: ~2.4 mcg per day

Adults over 50: 2.4 mcg per day
Older adults are advised to supplement with B12 where necessary due to the reduced stomach acid and intrinsic factor and/or medications.

Pregnant women: ~2.6 mcg per day
Increased demand due to supporting the development of the fetal brain and nervous system.

Breastfeeding women: ~2.8 mcg per day
B12 supports a child’s growth and neurological development through breastmilk.

Children
Infants (0–6 months): 0.4 mcg/day
7–12 months: 0.5 mcg/day
1–3 years: 0.9 mcg/day
4–8 years: 1.2 mcg/day
9–13 years: 1.8 mcg/day
14–18 years: 2.4 mcg/day

Vegans and vegetarians are advised to supplement with B12 as it is only present in animal foods.

People with MTHFR mutations are advised to supplement with active forms of B12 to support proper methylation.

Consult your healthcare practitioner regarding your B12 levels and appropriate supplementation.

Cyanocobalamin: typical supplement values are around 25-100 mcg daily. High doses (1,000–2,000 mcg/day) may be recommended for those with impaired absorption. Methylcobalamin:  Up to 1,000 mcg/day for general health or in cases of deficiency.

High doses of vitamin B12 of up to 1,000 mcg are considered safe because the body will use the amount that it needs, and any excess will be flushed out through urine.

A study comparing bioavailability of crystalline B12 at different oral doses found that the body can only absorb about 1-1.2 mcg B12 via the gut per dose. Small doses (around 2-3 µg) were absorbed at a rate of around 50%, and as the dose increased, the rate of absorption decreased.
Absorption rate of a 5 mcg dose was around 27%, and at 10 µg, only 15%. High-dose supplements were not found to proportionally increase absorption.⁵
In some cases, sublingual B12 supplementation may be more effective than oral administration.

Sublingual lozenges/drops are available as cyanocobalamin or methylcobalamin. Dosages may range from 100-1,000mcg daily.

Methyl-Life’s Active B12 product is one of the few that contains all three active forms of B12 (methylcobalamin, adenosylcobalamin, and hydroxocobalamin) in a sublingual tablet.

Sublingual B12 is absorbed directly through the mucus membranes in the mouth, bypassing the digestive tract and intrinsic factor. This is ideal for people with impaired absorption.

Intramuscular B12 injections are given as hydroxocobalamin or cyanocobalamin. Dosage is usually 1,000 mcg per injection. These are given every 1–2 weeks initially, then monthly depending on the severity and cause of deficiency.

B12 injections are advised for those with severe deficiency and/or impaired absorption due to 

pernicious anemia or surgery.

A high dosage of B12 is around 500-1,000 mcg a day, either given via oral supplements or B12 injections.  These dosages are typically given to treat deficiency resulting from pernicious anemia, gastrointestinal disorders (such as Crohn’s disease), malabsorption syndromes, gastric surgeries and long-term use of medications (such as proton pump inhibitors).

B12 is a water-soluble vitamin, so anything the body cannot use immediately is excreted in urine.

There is no upper limit (UL) because toxicity due to high dosage is unlikely. In general, high-dose B12 supplementation is safe.

Who may need high dosages of B12:

• People with absorption difficulties due to insufficient stomach acid, lack of intrinsic factor (pernicious anemia), or gastrointestinal surgery⁶
• Vegans/strict vegetarians
• People affected by certain genetic variants, e.g. MTR, MTRR, or TCN2 which impair how B12 is metabolized or delivered to tissues.