B12 dosage for adults

If you want a single number to put on a shelf and forget about, here it is: a 1,000 µg B12 pill twice a week covers every healthy adult vegan. This article explains why that number is right, what the alternatives are, and when you might pick something different.

The tl;dr

Two equally effective regimens for adults (ages ~19–64, no medical conditions):

• Daily: 25–100 µg of cyanocobalamin or methylcobalamin
• Weekly: 1,000–2,000 µg taken twice per week (say, Monday and Thursday)

Either works. Pick whichever you will actually remember.

Why the numbers are so different

The body absorbs B12 through two pathways:

1. Intrinsic-factor-mediated uptake in the terminal ileum, which caps around 1.5–2 µg per meal.
2. Passive diffusion across the gut wall, which picks up roughly 1% of whatever B12 is present above that cap.

At a 10 µg daily dose, almost all absorption is through pathway 1. Scaling the dose up means you plateau quickly on pathway 1 and start depending on the 1% passive absorption. That’s why a single 1,000 µg pill doesn’t deliver 1,000 µg usable — it delivers roughly 10 µg usable. Over a week, two 1,000 µg pills provide ~20 µg absorbed, which is more than enough.

The Academy of Nutrition and Dietetics’ plant-based diet guidance and the Vegan Society converge on the same math from different angles: either regimen above works.

Why not just eat fortified foods?

You can, if you’re disciplined. A consistent intake of fortified plant milks, nutritional yeast, and some cereals can meet daily needs. But this approach is fragile — switch brands, go on vacation, pick the wrong carton, and you drop below adequate. A dedicated supplement is 3¢ a day and eliminates that variability. Use fortified foods as a floor; use a supplement as insurance.

Which form?

• Cyanocobalamin — the default. Cheapest, most stable, most studied. Converts in the body to the active coenzyme forms.
• Methylcobalamin — an active form. Marketed as “natural.” Works fine; not meaningfully better for healthy adults.
• Hydroxocobalamin — the injectable form used clinically for severe deficiency. Unnecessary for prevention.
• Adenosylcobalamin — the other active form. Unnecessary for prevention.

If you are healthy, cyanocobalamin is honest and effective. If supplement marketing has made you anxious about it, methylcobalamin is fine. The full cyano vs methyl vs hydroxo comparison has more detail.

When to deviate from the default

• Pregnancy and breastfeeding — slightly higher needs. See B12 in pregnancy and breastfeeding.
• Over 50 — intrinsic-factor production drops with age across the whole population. A daily 100–500 µg supplement is a safer default than the once-a-week approach.
• On metformin, PPIs, or H2 blockers — these drugs can impair B12 absorption. Supplement daily and ask your doctor about testing.
• Recovering from deficiency — short-term aggressive dosing, often injections administered by a clinician. Do not self-manage active deficiency with oral supplementation alone.
• Infants and young children — pediatric doses differ. See B12 for vegan infants and children.

Is there an upper limit?

There is no established upper limit for B12. Excess is excreted in urine. Doses as high as 2,000 µg daily are used clinically with no reported toxicity. You cannot “overdose” on B12 at supplemental levels.

Common misconceptions

• “I need to match daily needs exactly each day.” You don’t. B12 is stored in the liver; a single large weekly dose works because of that storage plus passive absorption.
• “More is always better.” Not really. Above 2,000 µg per dose the marginal absorption is tiny. Spend the difference on lentils.
• “Methylcobalamin is the bioactive form and far superior.” Both active and inactive forms convert efficiently in healthy adults. Marketing has outrun evidence here.

The punchline

Pick the regimen you’ll actually follow. 25–100 µg daily, or 1,000–2,000 µg twice weekly. Cyanocobalamin is the honest default. Everything else is detail — and the details are covered in the B12 pillar article.