Vitamin B12

An essential vitamin for nerves and red blood cells. Shots treat deficiency; extra B12 hasn't been shown to boost energy or weight loss when levels are normal.

Evidence: Extensive clinical research
Half-life: Varies
Sources: 2 cited

Is vitamin B12 FDA-approved?

FDA-approved as a prescription injection; pills are sold as supplements.

Overview

B12 is needed to make red blood cells and keep nerves healthy. Deficiency causes anemia, tiredness, numbness and memory problems. It's more common in older adults, vegans, after weight-loss surgery and with long-term metformin or acid-reducer use. Shots or high-dose pills fix deficiency. Many clinics add B12 to weight-loss programs, but there's no good evidence it helps weight loss if your levels are already normal.

How it works

B12 helps two enzymes: one is needed to make DNA (and so new blood cells), the other helps the body process certain fats and proteins.

How long does vitamin B12 stay in the body?

The liver stores enough B12 to last years, so there isn't a simple dose-by-dose half-life.

What it's studied for

B12 deficiency and anemia
Nerve damage
Metformin-related deficiency

Reported side effects

Injection-site pain
Acne or rash (uncommon)
Allergic reactions (rare)

Sources

The studies and prescribing labels this page is written from.